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 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