BYG411 G IWE Project Assignment
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Welding and Heat treatment
of AISI 4130
Use of AISI 4130 steel in hydrogen sulfide environment and
its post weld heat treatment conditions.
DMITRI RYBAKOV
HELGE ANDREAS FALKUM
University of Agder (UIA), 2019
IWE Project assignment
IWE Project Assignment
Preface
This work is prepared as a part of written examination in accordance with IAB-252-07/SV-00. The framework
for this master thesis was given by company MHWirth, that specializes in offshore drilling. This work is partly
based on the Master's thesis “Post weld heat treatment of low alloyed steel pipes” written in spring 2019, but
it is completely independent and can be read separately.
We want to give special thanks to our teacher and supervisor Tor John Rødsås for all his support and
contribution to this work.
Would also like to thank engineering company Nymo, who have kindly providing us with material samples.
Author
Dmitri Rybakov
Helge Andreas Falkum
Kristiansand, November 2019
IWE Project Assignment
Summary
This work includes preparation and examination of test pieces from 3”, Sch XXS pipe, including hardness test.
Which gives a good foundation for the development of a welding procedure for AISI 4130 pipe of mentioned
size.
The result of this work is proposal of pWPS (preliminary welding procedure specification) for AISI 4130 steel
pipe with outside diameter 60,3 mm and wall thickness 11,7 mm. Bases for proposed pWPS is study of welded
samples, common practice, theory and discussion around material science and welding.
IWE Project Assignment
Table of content
Preface ........................................................................................................................................2
Summary .....................................................................................................................................3
Table of content ...........................................................................................................................4
1
Introduction ..........................................................................................................................6
2
Sample preparation...............................................................................................................7
2.1
2.1.1
2.1.2
2.1.3
2.1.4
2.1.5
2.1.6
2.1.7
2.1.8
2.1.9
3
3.1.1
3.1.2
3.2
3.2.1
3.2.2
3.2.3
4.1.1
4.1.2
4.2
4.2.1
4.2.2
4.2.3
4.2.4
6
Hydrogen sulfide damage ........................................................................................................ 13
Hydrogen cracking in a weld ......................................................................................................................... 13
Material hardness ......................................................................................................................................... 15
Recovery, recrystallization and grain growth ............................................................................ 15
Recovery ....................................................................................................................................................... 16
Recrystallization ............................................................................................................................................ 17
Grain Growth ................................................................................................................................................ 18
Case and materials .............................................................................................................. 19
4.1
5
Preparation ..................................................................................................................................................... 7
Preheating ....................................................................................................................................................... 7
SMAW Welding ............................................................................................................................................... 7
Polishing .......................................................................................................................................................... 8
Cutting............................................................................................................................................................. 8
Polishing .......................................................................................................................................................... 9
Etching .......................................................................................................................................................... 10
Hardness Testing ........................................................................................................................................... 10
Heat treatment ............................................................................................................................................. 11
Essential theory................................................................................................................... 13
3.1
4
Welded samples ........................................................................................................................ 7
Material .................................................................................................................................. 19
Cooling transformation diagram ................................................................................................................... 19
Material Data Sheet ...................................................................................................................................... 20
Mandatory requirements ........................................................................................................ 20
ASME B31.3 ................................................................................................................................................... 20
NACE MR0175 ............................................................................................................................................... 21
NORSOK M-601 ............................................................................................................................................. 23
ISO 14745 ...................................................................................................................................................... 23
Results and discussion ......................................................................................................... 24
5.1
Preheating effect..................................................................................................................... 24
5.2
PWHT ..................................................................................................................................... 25
PWPS proposal .................................................................................................................... 26
6.1
Thickness and allowances ........................................................................................................ 26
6.2
Weld passes ............................................................................................................................ 28
6.3
Heat input ............................................................................................................................... 29
6.3.1
6.3.2
6.3.3
6.3.4
Root pass ....................................................................................................................................................... 30
Hot pass ........................................................................................................................................................ 30
Filling and cover passes................................................................................................................................. 31
Controlling of chosen heat input .................................................................................................................. 32
IWE Project Assignment
6.4
Welding position ..................................................................................................................... 32
6.5
Heat treatment ....................................................................................................................... 33
6.5.1
6.5.2
6.5.3
6.5.4
6.6
7
Preheat temperature .................................................................................................................................... 33
Interpass temperature .................................................................................................................................. 34
PWHT temperature ....................................................................................................................................... 34
Heating and cooling rates ............................................................................................................................. 36
PWPS ...................................................................................................................................... 37
Attachments ....................................................................................................................... 38
7.1
Attachment 1 – Material Certificate ......................................................................................... 38
7.2
Attachment 2 – Material Data Sheet ........................................................................................ 40
7.3
Attachment 3 – 48.08 electrode ............................................................................................... 41
7.4
Attachment 4 – Heat input data ............................................................................................... 42
7.5
Attachment 5 – Hardness measurements ................................................................................. 43
7.6
Attachment 6 – WPQ for API 5L X52 pipe ................................................................................. 45
IWE Project Assignment
1 Introduction
The main task of this assignment is to get some insights into the welding procedures, reduce welding cost and
improve quality of WPS used for welding of AISI 4130 pipes. By the end of this assignment PWPS is proposed,
which the assignment is leading to and finalizing with.
The central parts of this work is study of different heat treatment effects on hardness of the material and
preparation of PWPS (Preliminary Welding Procedure Specification) for AISI 4130 pipe in hydrogen sulfide
contaminated services.
To prepare the reader to the reading, we do a short summary on the rapport structure:
1. Sample preparation chapter - describe process of sample preparation.
2. Essential theory – what is considered as essential theory for this paper.
3. Case and material chapter - describes material in the case and some theory around H2S damage.
4. Results and discussions – Discussing results from sample study and hardness measurements.
5. PWPS proposal – theory and discussion around PWPS proposal
Biography is dropped in this work, since in this work we only refers to national and international standards.
Because of restrictions in number of pages, this paper was somewhat squeezed but should not miss anything
essential for the reader.
IWE Project Assignment
2 Sample preparation
2.1 Welded samples
Engineering company Nymo kindly provided two 150 mm long pipe samples for this work.
Pipe samples are made of low alloyed carbon steel AISI 4130 with outer
diameter 60,33 mm (2 inches) and wall thickness 11,07 mm (Schedule:
XXS). Material certificate is attached, see Attachment 1 – Material
Certificate. As it is low-alloy steel, it has relatively good toughness,
weldability and machinability, although it requires to be heat treated
because of the relatively high chromium content.
Figure 1, Raw AISI 4130 pipe samples from Nymo
2.1.1 Preparation
Pipes groove was prepared with following parameters:
Groove angle: 60°C
Root gap: 3mm
Root face: 2mm
Figure 2, Butt Weld End Preparation
Pictures were taken by the author. License: CC BY-SA 3.0
2.1.2 Preheating
A digital thermometer was used to ensure that preheat
temperature of 120 °C was achieved, as in accordance with ASME
B31.3. Thermostat was also used to make sure that interpass
temperature stays just under 250 °C, as in accordance with Norsok
M-601.
Figure 3, Preheating with flame torch
Picture was taken by author. License: CC BY-SA 3.0
2.1.3 SMAW Welding
Choice of the electrode was made in accordance with the strength and composition requirements. Data on
filler material is attached, see Attachment 3 – 48.08 electrode. Electrode of 2,5 mm in diameter (slightly
smaller that root gap) was chosen for root gap to avoid incomplete filled groove and good penetration and 3,2
mm electrode was chosen for the rest of the passes.
Welding was performed in accordance with a WPQ for similar steel and same electrode, see Attachment 6.
Welding log with heat input data is attached, see Attachment 4 – Heat input data.
IWE Project Assignment
Figure 4, Pictures of welding process. Root and cap welding stages
Pictures were taken by the author. License: CC BY-SA 3.0
2.1.4 Polishing
After welding (and between every pass) slag is removed and welded area is polished to remove impurities and
prepare the surface for the next weld string.
Figure 5, Polishing of welded sample.
Pictures were taken by the author. License: CC BY-SA 3.0
2.1.5 Cutting
To expose the cross-section of the weld and to prepare sample pieces for hardness testing and microscopy,
welded pipe samples were cut out using a band saw:
Figure 6, Cutting of samples
Pictures was taken by author. License: CC BY-SA 3.0
IWE Project Assignment
2.1.6 Polishing
Polishing was done in two stages: milling and polishing with waterproof silicon carbide paper.
Milling was necessary to achieve flatness and parallelism of sample surfaces. ISO 6507-31 states that
maximum deviation in the flatness of the test and support surfaces shall not exceed 0,005 mm. The
maximum error in parallelism shall not exceed 0,010 mm in 50 mm long samples.
Figure 7, Use of milling mahine for surface correction and smoothing
Pictures were taken by the author. License: CC BY-SA 3.0
ISO 6507-3 further states that the test surface shall be free from scratches that interfere with the
measurement of the indentations. The test surface roughness, Ra, shall not exceed 0,05 μm. That can be
archived by using grinding paper with grind grade P2400 which gives surface roughness of 0.025 µm.
We perform polishing with help of polishing machine Knut Rotor in following steps: P320, P800, P1200, P2400:
Figure 8, Polished samples
Pictures were taken by the author. License: CC BY-SA 3.0
1
ISO 6507-3 standard specifies requirements for preparation of test piece for Vickers hardness testing.
IWE Project Assignment
2.1.7 Etching
For etching, nitric acid (also called Nital) was used.
Figure 9, Etching with 3% Nital, time from 1 to 6 is 6 seconds.
Pictures were taken by the author. License: CC BY-SA 3.0
At the end there was six samples that was prepared. Four from welded pipe without preheating and four from
welded pipe that was preheated with flame torch to 120 °C.
2.1.8 Hardness Testing
Hardness testing was done with hardness testing machine ZHU250CL, from Indentec. Calibration of this
machine was done 2,5 years ago, which it is less then recommended schedule of 12 month by ISO 6507-2.
However it should be sufficient given that the machine is seldom used and verification of overall performance
of the testing machine by means of calibrated reference blocks was performed in accordance with the
mandatory schedule.
Figure 10, Hardness testing machine
Pictures were taken by the author. License: CC BY-SA 3.0
IWE Project Assignment
Hardness survey is performed in accordance with ISO 15156-2. It is carried out using the Vickers HV10 and
impression was made as suggested by the standard, see Figure 11.
Figure 11, Butt-weld survey method for Vickers hardness measurement, ISO 15156-2.
Copied from ISO 15156-2 – 2015, Copyright International Organization for Standardization
2.1.9 Heat treatment
To achieve even heat distribution and heat/cooling rate of maximum 335 °C/h, as in accordance with ASME
B31.3. standard, a programmable furnace was used.
Figure 12, Controlled heat treatment in furnace
Pictures were taken by the author. License: CC BY-SA 3.0
The heating rate was around 250 °C/h, while the cooling rate could not be controlled in this furnace, thus
uncontrolled cooling in turned off furnace was used. The cooling rate was timed and noted for every 50 °C
(see Table 1), giving the following treatment diagrams:
IWE Project Assignment
Figure 13, PWHT diagrams
Made by author, License: CC BY-SA 3.0
Table 1, Cooling rate in furnace
Made by author, License: CC BY-SA 3.0
IWE Project Assignment
3 Essential theory
3.1 Hydrogen sulfide damage
Hydrogen Sulfide can be a major problem during a drilling process. Exposure to H2S-containing production
fluids can lead to cracking and sudden failure of metallic piping components.
In 1940s and early 1950s in West Texas the oil and gas industry had its first challenges with gas fields containing
H2S (sour gas). Further failures were reported in Canada. To solve the problem, the international organization
for corrosion control NACE, formed a committee to accumulate information and provide a solution to this
hydrogen sulfide issue and has developed one of its most important documents: NACE MR0175.
Cracking that can be caused by H2S, includes stress corrosion cracking, hydrogen-induced cracking and
stepwise cracking, stress-oriented hydrogen-induced cracking, soft zone cracking, and galvanically induced
hydrogen stress cracking.
Hydrogen-induced cracking (HIC)
Planar cracking that occurs in carbon and low alloy steels when atomic hydrogen ions diffuse into impurity
pockets of steel and then combines into molecular hydrogen at trap sites, expanding the pocket to a crack
formation.
Hydrogen stress cracking (HSC) and Sulfide Stress Cracking (SSC)
Cracking that results from the presence of hydrogen in metal and tensile stress (residual and/or applied).
Stepwise cracking (SWC)
Cracking that connects hydrogen-induced cracks on adjacent planes in steel.
Stress corrosion cracking (SCC)
Cracking of metal involving anodic processes of localized corrosion and tensile stress (residual and/or applied).
Does not involve hydrogen in its definition, but SCC can be accelerated in the presence of water and H2S.
Stress-oriented hydrogen-induced cracking (SOHIC)
Staggering small cracks formed approximately perpendicular to the principal stress (residual or applied)
resulting in a “ladder-like” crack array linking of Hydrogen induced cracks.
Soft-zone cracking (SZC)
Form of SSC that can occur when steel contains a local “soft zone” of low-yield-strength material. Does not
involve hydrogen in its definition, but hydrogen embrittlement can accelerate cracking in “soft zones”.
Galvanically induced hydrogen stress cracking (GHSC)
Cracking that results due to the presence of hydrogen in a metal induced in the cathode of a galvanic couple
and tensile stress (residual and/or applied).
3.1.1 Hydrogen cracking in a weld
HIC often occurs in arc welding, where hydrogen is discharged from moisture in the electrode, flux,
shielding gas or environment and contaminants containing hydrogen such as grease, oil, and cutting fluids.
IWE Project Assignment
Following can be done to lower hydrogen in the weld:
•
•
•
•
Pre-heating and post-heating the metal to allow the hydrogen to diffuse
Clean the base material prior to welding to eliminate organic materials
Use low hydrogen rods
With processes involving flux, make sure consumables are stored properly and not exposed to
moisture.
Figure 14, cracking process, due to hydrogen diffusion
Made by author, License: CC BY-SA 3.0
Figure 15, SEM micrograph showing SSC and HIC cracks
Copied from “Case Studies in Engineering Failure Analysis, Volume 1, Issue 3” CiteSeerx. Author: S.M.R. Ziaei, A.H. Kokabi,
M. Nasr-Esfehani. Case: ‘Sulfide stress corrosion cracking and hydrogen induced cracking of A216 -WCC wellhead flow control valve
body’.
IWE Project Assignment
3.1.2 Material hardness
Reduction of impurities in the material is an important step in prevention of cracks caused by hydrogen. Small
amount of impurities makes the material lattice compact, thereby reducing the amount of spots for hydrogen
gas to accumulate.
High hardness is often a result of high local stresses
that is caused by material impurities. Therefore
there is a limit on maximum allowed hardness of 22
HSR, in NACE standard.
Figure 16, Material impurities
CC BY-SA 3.0, Altered. Source: Wikimedia Commons
3.2 Recovery, recrystallization and grain growth
Residual stresses is a result of changes in microstructure that include changes in grain shapes as a result of
plastic deformation, strain hardening, increased dislocation density and impurity mechanisms. Material
strength and other properties like electrical conductivity and corrosion resistance can be modified as a result
of stored internal strain energy. Material properties can revert back fully or partially by appropriate heat
treatment or annealing.
Recovery of material can be described by two processes that take place at elevated temperatures: recovery
and recrystallization. Continues heating after recrystallization can trigger extensive grain growth and
weakening of the material.
Figure 17, Recrystallization steps of a metallic material
Adapted and transformed from Wikipedia, work of Daniele Pugliesi. License: CC BY-SA 3.0
IWE Project Assignment
3.2.1 Recovery
The mechanism of recovery is mainly motion of vacancys in microstructur. This involves a migration of point
defects to the grain boundaries or dislocations, and a combination of point defects. Recovery is the main
mechanism that releases the internal stresses in welded materials without decreasing the strength that was
acquired during material working and heat treatment.
As the material is heated during the recovery, the yield and tensile strength are reduced (see Figure 18),
allowing for the stored internal strain energy to be relieved by a dislocation movement.
Figure 18, Influence of annealing temperature on strength and ductility of brass.
Adapted from Fig 7.22, Material Science and engineering by W. D. Callister and D. G. Rethwisch 8e.
It is important to notes that although recovery significantly reduces the residual stresses in the material, it
does not get rid of them all. Leaving some of the most persistent ones left, that can further trigger nucleation
points at elevated temperatures.
Figure 19, Recovery process
Adapted and transformed from Wikipedia, work of Daniele Pugliesi. License: CC BY-SA 3.0
IWE Project Assignment
3.2.2 Recrystallization
As mentioned above, even after the recovery is complete, the grains are still in relatively high strain energy
state. In order to completely relive the material structure from residual stresses, the material must be heated
to the temperature of recrystallization2, TR.
An important thing to notice is that recrystallization temperature depends on many variables and is not a fixed
temperature similar to melting temperature or critical temperature of alloys. The increased annealing time
reduces the recrystallization temperature. Although the extent of a recrystallization process is dependent on
both time and temperature, nucleation will not be triggered under eutectic temperature unless there is
sufficient strain energy in the material to trigger nuclei and to drive their growth. Thus the driving force to
produce new strain-free grain structure is the difference in internal energy between the strained and
unstrained material.
Figure 20, Recrystallization process
Adapted and transformed from Wikipedia, work of Daniele Pugliesi. License: CC BY-SA 3.0
Recrystallization is easier in pure metals than in alloys and occurs at lower temperatures, at approximately
30% of melting temperature, while it requires as much as 70 % of melting temperature for high alloyed steels.
Thereby it can vary in the range of 0,3Tm – 0,7Tm, depending on the amount of alloying elements in steel [1].
The recrystallization temperature of pure Iron is 450 °C, thus increasing with the amount of alloying elements.
2
Recrystallization temperature is the temperature at which a specific material is recrystallized within approximately one
hour.
IWE Project Assignment
3.2.3 Grain Growth
After recrystallization is complete, the structure will be completely free from internal stresses, but the strainfree grains will continue to grow at the expanse of their neighbors if the material is kept at elevated
temperature. Large grains will grow at the expense of small ones.
Figure 21, Grain growth process
Adapted and transformed from Wikipedia, work of Daniele Pugliesi. License: CC BY-SA 3.0
The driving force for grain growth is the energy that is associated with grain boundaries. As grains increase in
size, the total boundary area decreases, leading to a reduction in the total energy of the structure. An
important fact is that grain growth does not require recovery and recrystallization prior to it.
IWE Project Assignment
4 Case and materials
This chapter includes relevant information on material data in chapter 4.1 and mandatory requirements in
chapter 4.2 that have to be considered when specifying a welding procedure for steel pipes that will be
exposed to hydrogen sulfide.
4.1 Material
Manufacturing process specified in the material certificate for our samples is illustrated with the TimeTemperature diagram in Figure 22.
Figure 22, TT diagram for AISI 4130 pipe
Made by author, License: CC BY-SA 3.0
4.1.1 Cooling transformation diagram
Company Timken Steel is one of the leading steel manufacturers. The company has developed isothermal and
continuous cooling transformation diagrams for SAE 4130 steel.
Figure 23, IT and CCT for SAE 4130 steel from timkinsteel to the left and simplified CCT diagram to the right
Copied from TimkenSteel.com. Copyright ©2014 TimkenSteel Corporation.
Right side of the figure is made by author, License: CC BY-SA 3.0
IWE Project Assignment
4.1.2 Material Data Sheet
Material Data Sheet (MDS) for AISI 4130 pipes used in high-pressure mud system in this assignment is X21,
developed by MHWirth, see Attachment 2 – Material Data Sheet.
Requirements from MDS X21:
✓ The maximum hardness shall be in accordance with the NACE MR0175 / ISO 15156 standard. That is
no more than 22 HRC or 250 HV.
✓ Fittings and pipes shall be delivered in the liquid quenched and tempered conditions. The tempering
temperature shall be a minimum of 650 deg C.
✓ Requirements to chemical composition specify restrictions to Sulphur and Phosphorous content of
less than 0,025%.
✓ Mechanical properties are specified to a minimum of the following values:
Minimum yield strength: Reh=>517 MPa ( 75 KSI), Minimum tensile strength: Rm =>690 MPa (100 KSI),
Minimum elongation: A5 => 17%, Minimum reduction of area: Z => 35%.
4.2 Mandatory requirements
Following requirements is considered mandatory:
•
•
•
Technical requirements of ASME B31.3 .
Welding and inspection of piping, Norsok M601.
Requirement for sour service (H2S) piping, NACE MR 0175 /ISO 15156.
The following chapters list relevant parts of applicable standards.
4.2.1 ASME B31.3
Preheat
According to ASME B31.3, chapter 330.1.1, alloy steel with Chromium content between 0,5% and 2%
(materials with P-No 4), require preheating temperature not less than 120 °C.
Table 2, Preheat Temperatures, ASME B31.3
Copied from ASME B31.3 – 2016, Table 330.1.1. Copyright ASME International
IWE Project Assignment
PWHT
Requirements for post weld heat treatment is given in table 331.1.1. According to which holding temperature
range is 650 – 705 °C and minimum holding time is set to 1/25 Hour/mm, but not less than 15 min.
Table 3, Postweld Heat Treatment, ASME B31.3 (2016)
Copied from ASME B31.3 – 2016, Table 331.1.1. Copyright ASME International
Interesting fact:
Previous revision of ASME B31.3 (2008) had different heat treatment temperatures for some material
groups. Like in the case of P-No 4, it was 704-746 °C. Which is just around lower critical temperature for low
alloyed steel ( A1 is somewhere around 720 °C ) which does not really make any sense. Some call it for ASME
trap that could in some cases lead to demolition of fabricated pipe spools.
Table 4, Postweld Heat Treatment, ASME B31.3 (2008)
Copied from ASME B31.3 – 2008, Table 331.1.1. Copyright ASME International
Heating and cooling rate
Chapter 331.1.4 states that the heating method shall be uniformly applied and may include an enclosed
furnace, local flame heating, electric resistance, electric induction, or exothermic chemical reaction. Above
315°C, the rate of heating and cooling shall not exceed 335°C/h divided by one-half the maximum material
thickness in inches at the weld, but in no case shall the temperature change rate exceed 335°C/h.
4.2.2 NACE MR0175
The consequences of sudden failures of metallic components in oil and gas industry is often associated with
their exposure to H2S-containing production fluids. Understanding of that led to the preparation of the first
edition of ANSI/NACE MR0175, which was published in 1975 by the National Association of Corrosion
Engineers, now known as NACE International. NACE MR0175 is covered by ISO 15156 standard and often refers
to as NACE MR0175/ ISO 15156.
IWE Project Assignment
ISO version splits the standard into three:
ISO 15156-1:
ISO 15156-2:
ISO 15156-3:
General principles for selection of cracking-resistant materials.
Cracking-resistant carbon and low alloy steels, and the use of cast irons.
Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys.
ISO 15156-1 is in a way an introduction part to ISO 15156 series and mainly clarifies terms, abbreviations and
general principles of material selection for sour service.
ISO 15156-2 specifies the requirements for testing methods and qualification of low alloy steels.
PWHT
A minimum PWHT temperature of 620 °C (1 150 °F) shall be used for low alloy steels.
Hardness testing
Hardness testing for welding procedure qualification shall normally be carried out using the Vickers HV 10 or
HV 5 method in accordance with ISO 6507-1, or the Rockwell method in accordance with ISO 6508-1 using
the 15N scale.
Weldments that do not comply with other paragraphs of this subclause shall be post weld heat treated after
welding. The heat treatment temperature and its duration shall be chosen to ensure that the maximum weld
zone hardness shall be 250 HV or 22 HRC.
Table 5, Acceptable hardness values for carbon-manganese steel, ISO 15156-2
Copied from ISO 15156-2 – 2015, Table A.1. Copyright International Organization for Standardization
Nickel content
Welding consumables:
Welding consumables and procedures that produce a deposit containing more than 1 % mass fraction nickel
are acceptable after successful weld SSC qualification by testing in accordance with Annex B.
Parent material:
Carbon and low-alloy steels are acceptable at 22 HRC maximum hardness provided they contain less than 1 %
mass fraction nickel.
IWE Project Assignment
4.2.3 NORSOK M-601
Interpass temperature
The interpass temperature shall be measured within the joint bevel. The minimum interpass temperature shall
not be less than the specified preheat temperature. The maximum interpass temperature shall not exceed the
maximum temperature during qualification or in no case above as stated below:
• 250 °C for carbon steels;
• 150 °C for stainless steels and nickel base alloys.
4.2.4 ISO 14745
This Technical Report provides recommendations for post-weld heat treatment (PWHT) of steels with
recommendations for holding temperatures and holding times for different materials and material
thicknesses.
According to Table 1 from the standard, holding temperature for AISI 4130 should be 620 – 680 °C. Holding
time is set to 60 min + material thickness in minutes.
Figure 24, PWHT parameters for steel. ISO 14745
Adopted from ISO 14745 – 2015, Table 1. Copyright International Organization for Standardization
Heating and cooling rate
In the case of PWHT in furnace, the temperature of the furnace at the time when the
product or component is placed in or taken out of the furnace should not exceed:
— 400 °C for simple products or components of uncomplicated shape and t < 60 mm thickness;
— 300 °C for complex products or components of complicated shape or t ≥ 60 mm thickness.
The rate for heating or cooling of the product or component should not exceed the following:
— for thickness t ≤ 25 mm: 220 °C/h;
— for thickness 25 mm < t ≤ 100 mm: (5 500/t) °C/h;
— for thickness t > 100 mm : 55 °C/h.
IWE Project Assignment
5 Results and discussion
5.1 Preheating effect
The minimum required preheat temperature of 120 °C (see table 1 in ASME B31.3) was used to preheat the
material prior to welding.
The most important reasons for the preheat is to
lower the cooling rate, producing more ductile and
less hard structure in heat affected zone and the root
pass of the weld. Thereby hardness measurements
can be a good indication on whether there is any
positive effect from the preheating.
Table of hardness measurements that was prepared
in accordance with ISO 15156-2 is attached, see part
1 of Attachment 5 – Hardness measurements. Only
data from bottom of the weld (root pass) was used to
generate the graphical representation.
Figure 25, Hardness measurements with and without
preheating to 120°C, from root and of the weld.
For unclear reasons in our case, preheat of 120 °C had
quite opposite effect on hardness the material, Made by author, License: CC BY-SA 3.0
making it harder.
Explanation of this unexpected effect of preheat could be that negative polarity was used for root pass
leading to high internal stresses and many more nucleation points which then developed into some grain
growth. That could lead to more grain boundaries and harder material. While the samples with no preheat
had smaller grain growth because of lower heat exposure, thereby retaining some of the original grain
structure.
However, this is just speculations and further discussion, Charpy testing and microstructure analysis is
required to allow for an explanation of this phenomenon.
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5.2 PWHT
Three different PWHT conditions were used, see chapter 2.1.9. The result from hardness measurement is listed
and graphically illustrated in following Figure 26.
Figure 26, Results from hardness measurements of heat treated samples
Made by author, License: CC BY-SA 3.0
According to ISO 15156, Individual HRC (Rockwell hardness) readings exceeding the value permitted by ISO
15156 may be considered acceptable if the average of several readings taken within close proximity does not
exceed the value permitted by ISO 15156 and no individual reading is greater than 2 HRC above the specified
value. Equivalent requirements shall apply to other methods of hardness measurement, which is Vickers in
our case.
2 HRC corresponds to approximately 14 HV in Vickers scale. Those the acceptable deviations from the 250 HV
allowing for single spices of hardness (according to NACE/ISO 15156-2) to 264 HV, as long as the average values
does not exceeds permitted limits of ISO 15156. That makes small deviations in hardness of sample 3.1 (PWHT
at 650 for 120 min.) fully acceptable since the average value is far below the accepted value of 250 HV.
Following values are listen In the green column of Figure 26:
✓ Base:
- Reduction of hardness in base material.
✓ HAZ:
- Reduction of hardness in heat effected zone.
✓ SP:
- Highest single point hardness value in the sample
✓ Average:
- Average hardness value in heat effected zone.
Red values indicates that value is not acceptable. Following can be said about each sample:
Sample 2.1, PWHT at 650 °C for 20 min:
Several single point values of hardness are above allowed 264 HV.
Sample 3.1, PWHT at 650 °C for 120 min:
Good results.
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Sample 4.1, PWHT at 705 °C for 20 min:
Hardness measurements revile extensive loss of hardness in heat affected zone, to a value below the one that
is specified in the material certificate. There is also some indication of a slight hardness reduction in the base
material for sample that was heat treated at 705 °C.
6 PWPS proposal
This chapter will discuss different parameters of a PWPS and how those parameters are set. Some simple
parameters are listed under, while more complicated parameters have been assigned separate subchapters
•
PWPS is based on EN ISO 15609-1 but also partly on ISO 15613 since we have performed sample
welding and hardness measurements.
•
Material is assigned group 5,1 as in accordance with ISO 15608 which corresponds to P-No 4 in ASME
BPVC Section IX.
•
Joint preparation is in accordance with ISO 9692-1.
6.1 Thickness and allowances
Thickness and diameter
Thickness and diameter is chosen with consideration of all tolerances in accordance with material standard
ASTM 519:
Figure 27, WT and OD tolerances from ASTM 519
Copied from ASTM 519. Copyright ASTM International
When defining thickness and diameter we must include fabrication allowances for thickness and outside
diameter. We want also include corrosion allowances to qualify this WPS for possible repair welds in the
future.
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Allowances for 3”, XXS (OD = 60,3, WT = 11,07):
✓ Thickness tolerances: +-12,5 % (See ASTM 519)
✓ Outside diameter tolerance: +-0,51 mm. (See ASTM 519)
✓ Errosion and corrosion allowance: 3 mm. (see DNVGL-OS-E101)
We add tolerances and allowances to our 3 inch pipe and following ranges for our WPS:
Thickness: 6,7 mm. – 12,4 mm.
Diameter: 57 mm. – 61 mm.
Throat thickness
When specifying actual throat thickness of butt weld, it should include tolerances and allowances for excess
weld metal (cap height) and excess penetration (root penetration). Those allowances are dependent on
chosen quality level and is specified in ISO 5817 standard:
Figure 28, Limits of imperfections, ISO 5817.
Adopted from ISO 5817. Copyright ISO
There is three quality levels to choose from, where quality level B corresponds to the highest requirement on
the finished weld. We will choose quality level C for our WPS, which gives us following throat thickness:
When defining through thickness we will have to account for both fabrication and corrosion tolerances here
as well.
Minimum throat thickness = minimum h (cap) + minimum h (root) + minimum material thickness =
0,5mm+0,5mm+(11,07mm * (100% – 12,5%) – 3mm) = 7,69 mm
Maximum throat thickness = maximum h (cap) + maximum h (root) + maximum material thickness =
7mm+4mm+(11,07mm * (100% + 12,5%)) = 23,45 mm
Range of 7,69 – 23,45 for throat thickness is very wide and will give little meaning to the welders, but it will
ensure sound well if minimum or maximum tolerances heats in.
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6.2 Weld passes
Root Pass
The common defect in a one sided butt weld is incomplete root penetration or root concavity. Increasing the
current may cause undercut defect in the root so the best strategy to eliminates lack of fusion and lack of
penetration defect in the root pass will be to choose straight polarity (DCEN, DC-).
For better control and good penetration with acceptable heat input we choose smaller electrode diameter,
2,5mm.
Hot Pass
Root pass should be grinned clean before hot pass which should be done as soon as possible to avoid hydrogen
cracking associated with the consumable. Not allowing root pass to cool before hot or filling pass will also
prevent cracking due to contractional stresses. A preheat is often required for the same reasons.
To normalize root pass and HAZ and to release residual stresses, a hot pass should be included in the welding
procedure. The polarity of hot and following passes should be DCEP to reduce amount of heat transferred to
the base material and to reduce inclusion defects as the electrons are liberated from the base metal surface,
not to it. Heat input should be slightly higher that the root pass to burn out any wagon tracks in the root pass,
which is a common problem in root passes. Some welders use hot pass over the root pass every time, whether
it needs it or not, which typically won’t hurt anything if it is done correctly and it will deal with any sloppy
grinding and cleaning of the root pass. The hot pass have higher current to allow for better penetration of root
pass without overheating the base material. This desired effect is to partly melt the root pass and push it out,
reducing the suck back (root concavity) effect that may have occurred on the ID of the pipe.
Since hot pass requires good penetration and adequate heat input, it is often done with the same electrode
size but that simce more propriate for hot passes that consists of two strings. Here we choose to use one string
for the hot pass which demands higher deposition rate, thus combining thicker electrode with reverse polarity
seems like a good idea. We choose thereby electrode diameter of 3,2mm.
Filling passes
Filling passes are meant for filling rather than digging, so the current can be reduced while keeping 3,2mm
electrode. Since the heat input is reduced, a slight side to side weaving could be used for filled passes
allowing for higher rate of filling and better fusion. However, not experienced welder can end up weaving
backwards which will lead to slag inclusions or end up with to much weaving which can be the cause of
undercut.
Cover passes
The cover passes are most exposed to hydrogen cracking because of fast cooling rate of the weld that
restricts escape of hydrogen, compared to previous passes. Faster cooling rate will also increase residual
stresses that may lead to cold cracking.
With interpass temperature and by not reducing the heat input on cover passes, we will keep the cooling
rate as high as possible, thus minimizing the danger of crack formations. Further improvement of safety in
the weld could be achieved by PWHT.
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6.3 Heat input
Calculation of heat input was done in accordance with EN 1011-1 standard:
Thermal efficiency for 111 welding process is 0,8 mm and travel speed was obtained by dividing the length of
the sample run by the elapsed time, as in accordance with ISO/TR 18491.
Figure 29, Sample runs of electrodes
Pictures were taken by the author. License: CC BY-SA 3.0
The travel speed is obtained by dividing the length of the weld (ROL) by the time it took to weld it.
For Ø2,5 electrode the ROL = 110 mm. and time = 55 sec. Thus welding speed was 2 mm/sec.
For Ø3,2 electrode the ROL = 210 mm. and time = 73 sec. Thus welding speed was 2,88 mm/sec.
The welding speed of such a vertical run will not be the same as for pipe, however it sufficient to get a
feeling of heat input that is required for future WPS.
Knowing the speed, we can apply the formula from EN 1011-1 to calculate heat input:
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6.3.1 Root pass
Sample run of Ø2.5 48.08 electrode, weldig current 80 A:
𝑄=𝑘∗
𝑈∗𝐼
22 𝑣 ∗ 80 𝐴
∗ 10−3 = 0,8 ∗
∗ 10−3 = 0,7 kj/mm
𝑣
2 𝑚𝑚/𝑠𝑒𝑐
We choose to use this heat input as the lowes value for root pass. Even though electrode supplier specifies
minimum current of 75 A, we keep the minimum at 80 A to ensure good penetration. To specify maximum
heat input we use 90 A current for 48.08 electrode. Also we allow for slight reduction in welding speed,
reducing it from 2 to 1,8 mm/sec. The maximum heat input would then be:
𝑄=𝑘∗
𝑈∗𝐼
22 𝑣 ∗ 90 𝐴
∗ 10−3 = 0,8 ∗
∗ 10−3 = 0,88 kj/mm
𝑣
1,8 𝑚𝑚/𝑠𝑒𝑐
The range of heat input for root pass is then 0,7 – 0,88 kj/mm.
6.3.2 Hot pass
Sample run of Ø3.2, 48.08 electrode, welding current is set 120 A:
𝑄=𝑘∗
𝑈∗𝐼
22 𝑣 ∗ 120𝐴
∗ 10−3 = 0,8 ∗
∗ 10−3 = 0,73 kj/mm
𝑣
2,88 𝑚𝑚/𝑠𝑒𝑐
The heat input for hot pass and rest of passes of ø3,2 electrode is approximately the same as for ø2,5
electrode that we are using on root pass. That does not correspond to what we have discussed previously in
chapter 6.2. However visual inspection of sample weld does not reveals any defects:
Figure 30, Cuts from the welding sample
Pictures were taken by the author. License: CC BY-SA 3.0
The reason for why such low heat input was sufficient is assumed to be that welder used straight polarity on
all passes, not only root pass.
On the next picture we can see root concavity at some parts of the weld, as a result of low heat input on the
root pass and too low heat input on hot pass:
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Figure 31, Root concavity on one of the samples
Pictures were taken by the author. License: CC BY-SA 3.0
We want to use DC+ for all passes after root pass with increased heat input. We keep 120 A as minimum
amperage for hot pass but will sufficiently reduce the travel speed:
𝑄=𝑘∗
𝑈∗𝐼
22 𝑣 ∗ 120𝐴
∗ 10−3 = 0,8 ∗
∗ 10−3 = 1,24 kj/mm
𝑣
1,7 𝑚𝑚/𝑠𝑒𝑐
For maximum heat input we specify current of 130 A and even slower welding speed:
𝑄=𝑘∗
𝑈∗𝐼
22 𝑣 ∗ 130𝐴
∗ 10−3 = 0,8 ∗
∗ 10−3 = 1,53 kj/mm
𝑣
1,5 𝑚𝑚/𝑠𝑒𝑐
The range of heat input for hot pass is then 1,24 – 1,53 kj/mm.
6.3.3 Filling and cover passes
For filling passes we reduce amperage to lowest values that is proposed by electrode vendor and increase
travel speed:
𝑈∗𝐼
22 𝑣 ∗ 110𝐴
∗ 10−3 = 0,8 ∗
∗ 10−3 = 0,92 kj/mm
𝑣
2,1 𝑚𝑚/𝑠𝑒𝑐
For highest heat input, use 120 A and slower travel speed:
𝑄=𝑘∗
𝑄=𝑘∗
𝑈∗𝐼
22 𝑣 ∗ 115𝐴
∗ 10−3 = 0,8 ∗
∗ 10−3 = 1,12 kj/mm
𝑣
1,8 𝑚𝑚/𝑠𝑒𝑐
The range of heat input for filling pass is then 0,92 – 1,12 kj/mm.
Cover passes can use same parameters as filling passes or slightly slower travel speed, thus increasing heat
input and reducing cooling rate. To reduce cooling rate and allow for more hydrogen to escape the weld, we
will also do the last cover pass in the middle of the weld.
Figure 32, Welding sequence
Made by author, License: CC BY-SA 3.0
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6.3.4 Controlling of chosen heat input
Knowing efficiency factor of 48.08 electrode (115 %), we can control our heat input by checking it with ISO
17671-2, which lists some recommendations for arc welding of ferritic steels. Though our steel is mainly
martensitic, ISO 17617-2 can be used since it covers low alloyed steels with similar chemical compositions.
Figure 33, Run out length for manual metal-arc welding
Copied from ISO 17617-2. Copyright International Organisation for Standarization
We can control our run out length with lengths in A.4.3 table. Firs we have to convert run out length (see
Figure 29) of our 350 long electrode, to what it would have been if we were to use 410 long electrode. It is a
simple calculation that gives us 129 mm for ø2,5 electrode and 246 mm for ø3,2 electrode. According to
Table A.4.3:
Heat input for our ROL of ø2,5 electrode is approximately 0,93 kj/mm
Heat input for our ROL of ø3,2 electrode is approximately 0,82 kj/mm
That does not seem to be far off from our choices of heat input for root, filling and cover passes. Heat input
for hot pass however is intentionally increased.
6.4 Welding position
Ideally we could create different WPS’s for different welding positions, making sure that optimal WPS
parameters are chosen for each position and skilled welders are used for hardest welds. However making a
separate WPS for each material and each position will lead to a lot of paperwork/filework and difficulties with
track of control. Too many WPS is seems to be a weak link in many organizations.
For reason of minimizing amount of WPS P number in ASME BPVC IX and group number in ISO 15608 was
created. To group together materials with similar properties from the welders perspectives.
Same with welding position. It is often specified one of the hardest positions to perform in order to qualify
other positions. However it does not seem as a good to specify one welding sequence for all positions. That is
because of management of melted puddle is much harder when it is effected by gravity, thus a single pass in
down hand position may require multiple passes overhead position. Different amount of passes will again
require different deposition rate and thereby different heat input parameters.
Therefore it may be a good idea to specify just hardest position in WPQ (welder performance qualification)
in order to qualify a welder for all other positions. However, WPS should not qualify all welding positions
with the same heat input and same welding sequence.
This pWPS will qualify all positions PH / PJ / PC, thus H-L045 and J-L045 and be of more universal type. However
it should be noted that it could be optimized as follows.
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For fabrication welds that allow for pipe rotation:
For fabrication welds that does not allow for pipe rotation and for filed welds:
6.5 Heat treatment
6.5.1 Preheat temperature
According to ASME B31.3, chapter 330.1.1, alloy steel with Chromium content between 0,5% and 2%
(materials with P-No 4), require preheating temperature not less than 120 °C (see Table 2 in chapter 4.2.1):
We can also check recommendations for minimum preheating temperature in ISO TR 17671-2.
Recommendation is based on heat input and maximum combined thickness.
Heat input for root pass = 0,88 kj/mm
Maximum throat thickness = 23,4 mm:
Figure 34, Conditions for welding steels with defined carbon equivalents (including CE for AISI 4130). ISO TR 17671-2, Fig A.2
Adopted from ISO 17617-2. Copyright International Organisation for Standarization
As we can see from Figure A.2 in ISO TR 17671-2, our heat input and maximum thickness parameters require
heat input between 100 an 125 °C.
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6.5.2 Interpass temperature
According to Norsok M-601, the minimum interpass temperature shall not be less than the specified preheat
temperature while the maximum interpass temperature for carbon steel shall not exceed 250 °C.
However we choose to disagree with the requirements to minimum interpass temperature not being lower
that rot pass temperature. That is because, even though all weld passes in a joint will have the same hydrogen
input, cooling capacity and composition, and therefore similar preheat (minimum interpass), there may be an
exception. That exception may be the root pass that due to contractional stresses may crack during cooling as
a result of material creep. To optimize conditions for root pass preheat temperature could be increased above
minimum interpass temperature. However, in case of AISI 4130 there is some amount of molybdenum that
makes it somewhat more creep resistant and we do not consider higher preheat temperature in our pWPS.
Note that preheat requirements from ISO TR 17671-2 may be much higher than requirements from ASME
B31.3 at low heat input and thick wall thickness. We therefore believe it to be a good practice to choose the
highest of preheat temperatures from ASME and ISO standard and increase it to closest 50°C, which then
should also be used as minimum interpass temperature for all weld passes.
In this case we choose following heat treatments during welding:
Preheat = 150 °C. To avoid hydrogen and hot cracking.
Minimum Interpass Temperature = 150 °C to avoid hydrogen cracking in the weld.
Maximum Interpass Temperature = 200 °C to control microstructure development.
Here we have chosen temperatures that we was most certain of, however it may be that such high
temperatures are unnecessary and is a waste of resources in case of AISI 4130 . That is because AISI 4130 is
somewhat creep resistant and preheat temperature (minimum interpass temperature) can be kept to a
minimum of 120 °C. While maximum interpass temperature are not so important since martensite formation
is not of any danger to martensitic AISI 4130 steel which will be PWHT after welding.
6.5.3 PWHT temperature
The heat from a welding process can cause localized expansion of HAZ in the base material, which is taken up
during welding by molten metal or base material. After welding, the material will start to cool and contract.
Since the heat during welding is not evenly distributed, some parts will contract more than others, leaving
some residual stresses behind. This type of impurities will make the material hard and brittle, thus require
treatment that can bring the material back to its design properties.
Post weld heat treatment (PWHT) is a process in which welded material is reheated to a temperature below
its lower critical transformation temperature, A1 and then it is held at that temperature for a specified amount
of time. The purpose of such heat treatment is to correct changes in microstructure after welding and to
reduce residual stresses in the material. Process of stress relieving is called recovery, see chapter 3.2.1
The process of PWHT consists of three steps: heating, holding, cooling. All steps have to be carefully planned
and controlled to get the desired outcome.
The rate of heating should be based on the thickness of the component and it is specified by the governing
codes. In the case with pressure piping, the most acknowledged design code is ASME B31.3. If the rate of
heating is not performed properly, either by heating too quickly or not evenly, temperature changes within
the component can become detrimental to the material, a specially if short holding time is used. As a result,
new residual stresses and stress cracks can be formed after the component is cooled to ambient temperature.
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Holding time is dependent on the alloy and material thickness. It can takes longer time for thick parts to
achieve homogenous temperature throughout the material, thus it should be taken in consideration when
deciding the holding time.
It is important to choose the PWHT temperature bellow the lower critical temperature, A1. To make sure that
all uncertainties are covered, there is a common practice to choose the PWHT temperature 50 °C below the
A1 temperature. Another important thing to consider is the original tempering temperature (tempering
temperature in the material certificate), which must not be exceeded. Otherwise, the mechanical properties
that is specified in the material certificate can be reduced, and in the worst case the material can become
useless. In such a case where PWHT temperature exceeds the manufacturer tempering temperature,
mechanical testing of the material must be performed after PWHT to make sure that the minimum design
values are not exceeded.
6.5.3.1 Choosing PWHT conditions
Before we proceed following should be noted. Tempering during manufacturing process is carried out
primarily to obtain mechanical properties required for the final application and to relieve residual stresses
from the hardening process. The actual tempering temperature is often chosen using trial and error method
but it should always be below critical temperature A1. However, for unknown reason, that is not the case
with our material sample that was supplied from Nymo AS, see material certificate in Attachment 1.
Thereby, we will not considering tempering temperature during manufacturing in our decision of PWHT
conditions. However, based on TT diagram (see Figure 22) and CCT diagram (see Figure 23) for our steel, we
can have in mind that, because of high tempering temperature and air cooling, it can contain some
untampered bainite that giving more brittle material compared to other AISI 4130 materials that was
tempered bellow A1 temperature during manufacturing.
Based on results from our hardness testing, see chapter 5.2. Accepted heat treatment was achieved for
sample 3.1, which was heated to 650 °C for 120 min.
We can control our values to common practice of having PWHT
temperature 50°C bellow material lowest critical temperature A1.
A1 temperature for low alloyed steels does not vary a lot and we
could use eutectoid temperature from standard Iron-Carbon phase
diagram, which is 727 °C. However, to be on the safe side we choose
to use a simulation program called Welding Note to find exact A1
temperature for our steel. The program was developed as an aiding
tool for welders to clarify welding parameters for the welding
procedures, see Figure 35. Welding note sets A1 temperature to
720°C for our material composition.
Applying common practice and subtracting 50 °C from lowest critical
temperature, we get 670 °C as suggested highest PWH temperature
based on common practice.
Further, based on requirements for post weld heat treatment in Figure 35, Welding parameters for AISI
ASME B31.3, PWHT temperature range should be between 650 - 4130
705°C, see Table 3 in chapter 4.2.1.
Made by author, License: CC BY-SA
3.0
To comply with both design standard and common practice, we can
state that PWHT temperature for our steel should be somewhere between 650 °C and 670 °C.
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Further testing is required to find the optimal holding time, which according to ASME B31.3 should not be less
than 1/25 Hour/mm, and in never less than 15 min. Which in our case can be calculated as follows:
60𝑚𝑖𝑛
∗ 11,07𝑚𝑚 = 26,6 𝑚𝑖𝑛 ≈ 30 𝑚𝑖𝑛
25𝑚𝑚
Based on that and our results we can conclude that holding time should be between 30 min and 120 min.
Ideally we would love to prefer further hardness measurements to find the optimal PWHT conditions,
considering 30 min holding time for 650 °C and then if insufficient for 660°C and 670 °C. As of now we choose
to go with 650°C and 120 min holding time.
6.5.4 Heating and cooling rates
Chapter 331.1.4 in ASME B31.3 states that the heating method shall be uniformly applied and may include
an enclosed furnace, local flame heating, electric resistance, electric induction, or exothermic chemical
reaction. Above 315°C, the rate of heating and cooling shall not exceed 335°C/h divided by one-half the
maximum material thickness in inches at the weld, but in no case shall the temperature change rate exceed
335°C/h.
Maximum material thickness at the weld (throat thickness) is set to 23,4 mm, so the temperature change
rate cat be calculated as follows:
23,4 𝑚𝑚
25,4 𝑚𝑚
𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑐ℎ𝑎𝑛𝑔𝑒 𝑟𝑎𝑡𝑒 = 335 °𝐶/ℎ ∗ (
) = 154 °𝐶/ℎ
2
We set temperature change rate to 150 °C/h or 2,5 °C/min in our PWPS, which is attached in chapter 6.6.
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6.6 PWPS
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7 Attachments
7.1 Attachment 1 – Material Certificate
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7.2 Attachment 2 – Material Data Sheet
MDS X21
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7.3 Attachment 3 – 48.08 electrode
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7.4 Attachment 4 – Heat input data
Heat input based on actual welding
Approximately only 30% of ø2,5 electrode was used, and 20% of ø3,2, starting with the new
electrode one each time.
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7.5 Attachment 5 – Hardness measurements
Part 1 – Effect of preheating
Comparison of material hardness with and without preheating
Graphical representation of preheating to 120 °C.
Average values from the bottom of the weld of all three samples was used to generate following graph.
Part 2 – Effect of PWHT
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Comparison of material hardness after different PWHT temperatures.
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7.6 Attachment 6 – WPQ for API 5L X52 pipe