Presentation on theme: "Hardness Testing Indentation Hardness used for steel – as opposed to scratch or rebound hardness It is indicative of ultimate tensile strength – Atoms."— Presentation 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
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.
ASTM and ISO use the HB value. It can be HBS (Hardness, Brinell, Steel) or the HBW (Hardness, Brinell, Tungsten) HBW = BHN Sometimes written as HBW 10/3000 (Tungsten, 10 mm diameter, 3,000 kg force)
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.
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
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