Hardness testing

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 2P

D D D2 d2

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 valuesMaterial Hardness

Softwood (e.g., pine) 1.6 HBS 10/100

Hardwood 2.6–7.0 HBS 1.6 10/100Aluminum 15 HBCopper 35 HBMild steel 120 HB

18-8 (304) stainless steel annealed 200 HBGlass 1550 HB

Hardened tool steel 1500–1900 HB

Rhenium diboride 4600 HB

Rockwell Hardness

Rockwell Hardness

Rockwell Hardness Scales

Scale Code Load Indenter Use

A HRA 60 kgf 120° diamond coneTungsten carbide

B HRB 100 kgf 1/16 in diameter steel sphere Al, brass, and soft steels

C HRC 150 kgf 120° diamond cone Harder steelsD HRD 100 kgf 120° diamond cone

E HRE 100 kgf 1/8 in diameter steel sphere

F HRF 60 kgf 1/16 in diameter steel sphere

G HRG 150 kgf 1/16 in diameter steel sphere

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 three-cornered 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 free-machining 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 work-hardened 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