14-01-2013, 04:54 PM
Hardness Testing
1Hardness.ppt (Size: 1.25 MB / Downloads: 292)
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.
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
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.
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