International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 4, Issue 4, April 2015
ISSN 2319 - 4847
An Overview on Effect of Preheating on Cold
Cracking of Low Alloy steel and Stainless Steel
Weld Joint
Merchant Samir Y
Lecturer, Fabrication Technology Department, Sir. B.P. Institute, Bhavnagar,
ABSTRACT
Weldability of alloy steel and stainless steel is having major consideration of hydrogen induced cracking or cold cracking.
During welding metal is heated above melting point and cooled at very high speed. So contraction forces are developed in weld
joint and HAZ. The microstructure of weld joint is also different in various zones of weld joint. Production of hard and brittle
microstructure like martensite, high carbon and other alloying elements present in parent metal, restraint in welding and
diffused hydrogen develop the hydrogen induced crack in various areas of weld metal and HAZ. HIC cracks are generally sub
surface and difficult to detect but sometimes it propelled towards the surface also. HIC crack is observed both in weld metal
and HAZ. Effect of preheating on cooling rate and alloying elements on development of HIC in low alloy steel and stainless
steel metal welding is reviewed in this paper. It was found that by applying preheating, cooling rate of weld metal is reduced
and solidification time is increased. Cold cracking can be reduced by selecting proper preheating temperature which depends
on carbon equivalent, diffusive content of hydrogen, weld heat input, joint restraint, yield strength of weld metal and thickness
of parent metal.
Keywords – Preheating, Cold Cracking, Low Alloy Steel, Stainless Steel
1. INTRODUCTION
Today in fabrication industry, welding is widely used as metal joining process. Different metals can be easily joined by
using welding process. There are many welding processes used in industry. Welding produced sound, high strength and
leak proof metal joint. Welding is producing joint by melting of parent metal and filler metal. The strength and
soundness of welding joint depends on many factors. Alloy steel and stainless steel are widely used in power plant, ship
building, process industry, cryogenic requirement, high temperature service requirement and dairy industries. These
metals are having different welding metallurgical problems due to different alloying elements. One of them is hydrogen
induced cracking, which occurs at very low temperature even long duration after completion of welding. It occurs in
stressed and non-stressed condition also. Because of rapid cooling a hard microstructure formed in weld metal and
HAZ when exposed to a hydrogen containing atmosphere, a crack is produced [1]. The low alloy steel is classified in
three types i.e. high strength low alloy steel, quenched and tampered low alloy steel and heat treatable low alloy steel.
Chemical composition of high strength low alloy steel varies as per grade of metal. Table-1 shows the range of alloying
elements available in SA568/SA568M high strength low alloy steel [2].
Table-1: Chemical Composition of SA 568/SA 568M [2]
Element
C
Ni Cr
Mn
Mo V
Cu Al
Col./Nb Si
S
P
Ti
N
percent
0.15- 1
0.9 0.60.2 0.1 1
0.1 0.1
0.3- 0.3- 0.3- 0.15 0.03
0.8
1.65
0.6 0.6 0.6
High strength low alloy steel is generally designed to provide higher strength than simple plain carbon steel. HSLA is
having C, Mn, Si like carbon steel but additionally it contains Nb, V and Ti which ensures precipitation hardening and
grain refinement. These alloying elements are strong carbide and nitride former. So they reduce grain size and increase
its strength and toughness together. The quenched and tamper alloy steel is having carbon less than 0.25% and other
alloying elements less than 5%. In this metal low carbon content is desirable due to minimize the hardness of
martensite and increase the martensite start temperature. The heat treatable low alloy steel contains carbon 0.25 – 0.5
% and other alloying element up to 5%. The higher carbon percentage increases hardness of weld joint and reduce its
toughness. All the low alloy steels are susceptible for hydrogen induced cracking hence preheating and interpass
heating is required before and during welding. Thermal cycle of heat treatable low alloy steel is shown in fig-1. Before
welding this metal is pre heated nearby up to martensite finish line and then welding is done. After welding, metal is
allowed to cool up to preheating temperature and then followed by PWHT nearby up to lower critical temperature line
A1. Due to this complete austenite will transform in martensite and then during PWHT it will transform into tempered
martensite [3].
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Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 4, Issue 4, April 2015
ISSN 2319 - 4847
Figure-1 Thermal history of heat treatable alloy steel (A) desired cycle (B) undesired cycle [3].
There are five types of stainless steel i.e. austenitic stainless steel, ferritic stainless steel, martenstitic stainless steel,
precipitation hardening stainless steel and duplex stainless steel. All the stainless steel are considered as high alloy
steel which contains alloying element more than 20%. All the stainless steel contains chromium 11-20% and Ni 0.5-11
%. Chemical composition of stainless steel is varies as per different grade. Table-2 shows the range of alloying
elements of SA 240 grade 405 ferritic stainless steel [4]. Ferritic and martenstitic stainless steel are specifically prone to
cold cracking. So preheating is required for ferritic stainless steel especially when high chromium content and high
parent metal thickness. Generally preheating temperature is set in range of 150°C to 250°C. For martenstitic stainless
steel no preheating is required when carbon percentage is below 0.1%. For carbon percentage between 0.1 to 0.2 %
preheating temperature set between 200°C to 300°C [5].
Element
percent
Table-2 Chemical Composition of SA 240 grade 405 [4]
C
Mn
P
S
Si Cr
Ni
0.08 1
0.0
0.03 1
11.5-14.5
0.6
4
Al
0.1-0.3
2. FUNDAMENTAL OF PREHEATING
Preheating is heating of complete parent metal or area nearby the weld joint before welding up to a specific
temperature. Preheating is used for following reasons [6][7]:
1. It reduce cooling rate of weld joint hence produce more uniform microstructure.
2. It provides resistance to cracking.
3. It decreases the cooling rate, hence allowed hydrogen to diffuse out harmlessly. So decreases the chances of HIC.
4. It reduced shrinkage stresses in weld metal and parent metal.
5. It increase temperature of steel below which brittle fracture will occur.
There are many factors affecting preheating temperature like code requirement, thickness of parent metal, chemical
composition of parent metal, use of fixture or restraints ambient temperature, filler metal composition especially
hydrogen content and welding history of the parent metal [6]. As per structural welding code D1.1/1.1:M2010,
preheating temperature can be obtained by two methods. In first method hardness of HAZ is controlled by controlling
cooling rate. The critical cooling rate for any metal is not depend on carbon but depends on carbon equivalent. Carbon
equivalent is the formula which represents the effect on alloying element on weldability of steel. There are many carbon
equivalent formulas given by different researchers but formula given by IIW is very famous which as in equation-1 [8].
As per AWS D1.1 when C% ≥ 0.11, CE(IIW) is selected and for C% < 0.11 Pcm is selected. Applicable range of CEN
is wider than CE and Pcm because of flexibility in selection values of A(C). The value of A(C) is so selected than it
approaches CE in high carbon region and Pcm in low carbon region [8]. Form critical cooling rate minimum value of
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Volume 4, Issue 4, April 2015
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preheating is calculated. The table-3 represent the required preheat temperature for different range of carbon equivalent
[9]. The relation of preheat temperature v/s carbon content and carbon equivalent is shown in fig-2.
Table-3 Preheat Temperature For Different Range of CE [9]
Carbon equivalent
Required preheat temperature range
Upto 0.3
Preheat is optional
0.3 to 0.45
Preheat 93°C to 205°C
Above 0.45
Preheat 204°C to 371°C
(A)
(B)
Figure-2 (A) Relation between preheating temperature and carbon percentage [10],
(B) Relation between preheating temperature and carbon equivalent [3]
Cooling rate of weld joint is increased with increase in the thickness of the plate. So HAZ of weld joint becomes harder
due to martensite formation. This hard HAZ is more susceptible to HIC. Transverse weld crack may observed in HAZ.
Fig-3 shows the relation between preheating temperature and thickness of plate for specific carbon equivalent [11][12].
Figure-3 Relation between carbon equivalent and thickness of plate [11]
The second method is controlling hydrogen content of weld joint. The basic steps of this method are calculate
composition parameter, calculate susceptibility index and determine minimum preheat temperature. This method is
more useful in welding of heat treatable alloy steel. Preheating can be applied by heating a small job in furnace or in
case of large job by electric strip heating, gas burner heating, radiation heating or induction heating. As per AWS D1.1
preheating must be applied to 3 inch (75 mm) distance from the weld center line in both directions for complete
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thickness of the parent metal [13]. For most of the HSLA steel preheating temperature up to 10°Cfor 1 inch, 50°C for 2
inch and 75°C for 3 inch plate thickness. For quenched and tempered low alloy steel preheating temperature required is
50°C for 0.5 inch, 100°C for 1 inch, 150°C for 1.5 inch plate thickness. Further increase in preheat temperature reduce
cooling rate and convert austenite into bainite or pearlite. Due to this both strength and toughness are decreased. For
heat treatable low alloy steel preheating temperature required is 200°C for 0.5 inch, 250°C for 1 inch, 300°C for 2 inch
plate thickness [3].
3. COLD CRACKING / HYDROGEN INDUCED CRAKING
Hydrogen comes in welding due to combustion product in oxy-fuel welding, decomposition product of cellulose type
electrode in SMAW welding process, moisture or oil on surface of work piece or electrode, moisture in flux, electrode
covering or shielding gas, and inadequate shielding of weld metal. Hydrogen reduces tensile strength of weld, makes
arc erratic and develops hydrogen induced crack in HAZ [14].
HIC develops in weld due to following reasons [6][10][15]
1. Presence of hydrogen in weld metal and parent metal
2. Martensite formation in weld metal and HAZ
3. High welding contraction stresses due to joint restraint and high thermal severity
4. Relatively low temperature between -100°C to 200°C
5. High welding travel speed
6. Low welding current
7. Insufficient cross section area of weld
8. Heat input
9. Parent metal thickness
Martensite is hard and brittle microstructure is susceptible for hydrogen cracking. Formation of martensite is at low
temperature this cracks are also known as cold cracking or delayed cracking. In low alloy steel weld metal is more
susceptible to HIC than HAZ [3][10][16][17][18]. Fig-4 shows Graville diagram for steel susceptible for HIC. In zone –
I which is low carbon and lo alloy steel is less susceptible to cold cracking, which develops due to high hydrogen
content and restraint. For alloys falling in zone – II, hardness varies with cooling rate. In this region alloy cooling rate
can be reduced by preheating. Zone-III is for high carbon and high alloy steel. Harden ability of these alloys is very
high. To reduce hardness cooling rate must be controlled [5][9][19][20].
Figure-4 Graville diagram for classifying steels according to their crack susceptibility [5][9]
4. EFFECT OF ALLOYING ELEMENTS ON CRITICAL PREHEAT TEMPERATURE
& HIC
During an experiment on three alloy steel having different alloying content, it was observed that with increase in
alloying content HIC susceptibility would increase due to increase in hardenability of weld metal. With increase in
alloying element diffusive hydrogen content would decreased. Diffusivity and solubility of hydrogen is strongly depends
on trapping of hydrogen due to various defects like dislocation, grain boundaries, matrix-particle interface etc. These
defects have affinity towards hydrogen results in trapping of hydrogen. Hydrogen atoms trapped were released only by
heating at high temperature and do not diffuse in metal, hence reduce susceptibility of HIC. With increase in alloying
element diffusivity of hydrogen decreased but solubility of hydrogen is increased. With increase in solubility of
hydrogen in weld, susceptibility of HIC is increased. Fig-5 shows the relation critical preheat temperature and hydrogen
diffusivity of alloy steel. It is clear that with increase in alloying element, though hydrogen diffusivity decreased but
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high critical preheat temperature required [21][22]. With increase in carbon equivalent increases the acicular ferrite in
HSLA steel welding. With increase in acicular ferrite, crack susceptibility of alloy steel increases [23]. So Ni up to
1.5% is useful to reduce the cold cracking [24]. In austenitic stainless steel HIC is produced due to diffusion and
enrichment of atomic hydrogen instead of hydrogen induced martensite [25].
Figure-5 variation of preheat temperature with diffusive hydrogen content [12]
5. EFFECT OF PREHEATING ON COOLING RATE, SOLIDIFICATION TIME AND HIC
During experiment on mild steel specimen it was found that cooling rate and solidification time depends on heat input
rate. The heat input is depending on arc voltage, welding current and welding speed. With increase in heat input rate
cooling rate decreased [26]. The heat input, cooling rate and solidification time can be calculated by using following
formulas [16][26][27].
Where
V = arc voltage
A = welding current
S = welding speed or arc travel speed (mm/min)
But for MMAW process the heat transfer efficiency is 0.65 to 0.85
Cooling rate can be calculated by using following formulas [14][27]
Step-1 first calculate the relative plate thickness t1
Where t = plate thickness
ρ = density of material g/mm3
C = specific heat of solid material J/g℃
ρC = volumetric specific heat J/ mm3 ℃
Tc = temperature near the pearlite nose on TTT diagram
Hnet = Heat input J/min
Step-2 If t1 > 0.75 then
R= cooling rate ℃/sec
K = thermal conductivity J/mm,s℃
To = Initial temperature of plate to be welded ℃
Step-3 If t1<0.75 then
Where
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Solidification time St can be calculated by using following formula [14].
Where L = Heat of fusion, J/mm3
Tm = Liquidus temperature of metal to be welded
By keeping welding current, arc voltage and welding speed constant if preheating temperature can be changed then its
effect can be calculated and shown in table-4. Fig-6 shows the effect of increasing preheating temperature on cooling
rate and solidification time. It is clear that with increase in preheat temperature cooling rate decrease rapidly and
solidification time increases. So chances of HIC decreased by increasing preheat temperature. With increase in preheat
temperature HIC failure life and stress bearing capacity can be increased of HSLA steel as shown in Fig-7 [3].
Table-4 Theoretical Calculated Values Of Cooling Rate And Solidification Time
Current
Amp
Arc
Voltage
Welding
Speed
Mm/Sec
Hnet
J/Mm
Plate
Thk mm
To
Cooling
Rate
°C/Sec
Solidificatio
n Time St.
Sec
Tc
90
21
1.52
1060
10
550
50
12.88
0.84
90
21
1.52
1060
10
550
100
9.39
0.90
90
21
1.52
1060
10
550
150
6.60
0.97
90
21
1.52
1060
10
550
200
4.42
1.04
90
21
1.52
1060
10
550
250
2.78
1.12
Figure-6 Effect of preheat temperature on cooling rate and solidification time
Figure- 7 Effect of preheating on HIC of HSLA steel [3]
6. CONCLUSION
From the review of the research work carried out by many researcher following major points are concluded:
1. Besides carbon, other alloying elements are also affecting the weldability of low alloy and stainless steel. Carbon
equivalent is one of the parameter affect the weldability of steel.
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2. Preheating can minimize the hydrogen induced cracking. Preheating temperature is selected by considering carbon
equivalent, thickness of parent metal and restraint available.
3. Preheating is reducing the cooling rate of weld joint and increasing the solidification time. So hard and brittle
martensite formation can be avoided which is one of the prime cause of hydrogen induced crack.
4. Diffused and solute hydrogen makes the weld metal very hard. With increase in alloying elements diffused
hydrogen content decreased but crack susceptibility is increased due to increase in its solubility. By selecting proper
preheating temperature, enough time is given to hydrogen to diffuse away from weld metal.
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
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[26] Merchant Samir Y., “Investigation on Effect of Heat Input on Cooling Rate and Mechanical Property (Hardness)
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AUTHOR
Samir Y. Merchant is working as a senior lecturer in Fabrication Technology Department, Sir
Bhavsinhji Polytechnic Institute, Bhavnagar, Gujarat, India since 2004. He obtained M.E.
Mechanical Engineering
(Production Engineering) from M.S. University, Vadodara with First
Rank in 2010. He has published four papers in international journals.
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