Transcript
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
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
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 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
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
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.
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)
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
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