Hardness Testing
• Indentation Hardness used for steel
– as opposed to scratch or rebound hardness
• It is indicative of ultimate tensile strength
– Atoms move out of the way to create indentation
• Two main types: Brinell and Rockwell
Brinell Hardness
Brinell Hardness
• A spherical indenter (1 cm diameter) is shot
with 29 kN force at the target
• Frequently the indenter is steel, but for harder
materials it is replaced with a tungsten
carbide sphere
• The diameter of the indentation is recorded
• The indentation diameter can be correlated
with the volume of the indentation.
Brinell Hardness
BHN 

D D 
2P
D
2
d
2

Brinell Hardness
• ASTM and ISO use the HB value. It can be HBS
(Hardness, Brinell, Steel) or the HBW
(Hardness, Brinell, Tungsten)
• HBW = 0.102 BHN
• Sometimes written as HBW 10/3000
(Tungsten, 10 mm diameter, 3,000 kg force)
Typical HB values
Material
Softwood (e.g., pine)
Hardwood
Aluminum
Copper
Mild steel
18-8 (304) stainless steel annealed
Glass
Hardened tool steel
Rhenium diboride
Hardness
1.6 HBS 10/100
2.6–7.0 HBS 1.6 10/100
15 HB
35 HB
120 HB
200 HB
1550 HB
1500–1900 HB
4600 HB
Rockwell Hardness
Rockwell Hardness
Rockwell Hardness Scales
Scale Code
Load
Indenter
Use
120° diamond cone
Tungsten
carbide
A
HRA 60 kgf
B
Al, brass, and
HRB 100 kgf 1/16 in diameter steel sphere soft steels
C
D
HRC 150 kgf 120° diamond cone
HRD 100 kgf 120° diamond cone
E
HRE 100 kgf 1/8 in diameter steel sphere
F
HRF 60 kgf
G
HRG 150 kgf 1/16 in diameter steel sphere
1/16 in diameter steel sphere
Harder steels
Conversion/Comparison
HBW
10/3000
HRA 60KG
HRB 100KG
HRC 150KG
Tensile
Strength
(Approx)
638
80.8
-
59.2
329,000
578
79.1
-
56
297,000
461
74.9
-
48.5
235,000
375
70.6
-
40.4
188,000
311
66.9
-
33.1
155,000
241
61.8
100
22.8
118,000
207
-
94.6
16
100,000
179
-
89
-
87,000
149
-
80.8
-
73,000
111
-
65.7
-
56,000
Effect of Strain Rate
Effect of Strain Rate
Effect of Temperature
Creep
• When a material is loaded below the yield stress
point for a long period of time, it may incur
plastic deformation.
• When the material is stretched below the yield
point at increased temperatures creep will
develop over several stages.
• The temperature level at which creep will initiate
depends on the alloy
– For aluminum, creep may start at approx. 200°C and
for low alloying steel at approx. 370°C
Creep
Creep
Effects of Punching Holes/Shearing
• Holes and shearing cause cold work near the
edges of the material.
• Cold work can lead to brittle failure/cracking
Drilling Holes
• The work hardening effect when drilling the
austenitic stainless steel grades eg 304, 316 is
the main cause of problems.
– make sure that the steel is fully annealed when
deep or small diameter holes are to be drilled.
– Cold drawn bar products should be avoided.
• rigid machines and tooling should be used
when drilling or reaming.
Drilling
• Center punching with conventional conical
shaped punches can result in enough localized
work hardening to make drill entry difficult.
– drill tip can deflect or wander, glaze the surface or
blunt the drill tip and result in drill breakages
• Where a punch mark is needed to help get the
hole started, a light mark using a threecornered pyramid tip punch is a better idea.
Drilling
• Essential to maintain feed rate to cut the work
hardened layer generated as the metal is cut.
– Dwell or rubbing must be avoided.
– Entry and re-entry should be done at full speed
and feed rate.
• When drilling through-holes, a backing plate
should be used to help avoid drill breakages as
the drill comes out of the blind side of the
hole.
Drills
The cutting angle should be
around 135°. Larger angles
produce thinner chips that
should be easier to remove,
which is important when
drilling stainless steels.
Lower angles of around 120°
can be used for drilling freemachining grades
Reaming
• Cold working during drilling, punching or
machining the preparation hole prior to
reaming austenitic stainless steels must be
minimized.
• Sufficient material must be left on the hole
wall however to allow a positive reaming cut
to be made to undercut the new workhardened layer produced.
Reaming
Shearing Steel
Shearing Steel
• If shear edges are to be left exposed, at least
1/16 inch of material should be trimmed
– Usually by grinding or machining
• Note that rough machining (edge planers
making a deep cut) can produce same effects
Effects of Welding
• Failures in service rarely occur in a properly
made weld.
– When failure occurs it is initiated at a notch defect
– This could come from flaws in the weld metal
• Welding-arc strikes may cause embrittlement
in the base metal
• Preheating before welding minimizes risk of
brittle failure.
– Less likelihood of cracking during cooling
Welding
• Rapid cooling of weld can have bad effects.
– If there is an arc strike with no deposited metal, it
will cool quicker than the rest and likely embrittle
• Welds are sometimes peened to prevent
cracking and distortion.
• Some specs prohibit peening in first and last
weld passes.
– Peening reduces toughness and impact properties
(work hardens the weld)
Single pass weld
Multipass weld
Defect
Thermal Cutting
• Oxyfuel, air carbon arc, plasma arc
• Similar problems with welding
– Pre-heating is desired in many applications
• Roughness of cut surface depends on
– Uniformity of pre-heat
– Uniformity of the cutting velocity
– Quality of steel
Thermal Cutting
Residual stress flame cut