The size of the indentation must be suitable for the size of the feature being measured. Anyone who is burning steel samples just doesn't have a clue. I have no concerns about metallographic preparation on ferritic steels. (Call me obsessive, but I have witnessed too much shoddy lab and field hardness testing.) I have reference blocks, but I want to obtain this correlation on material that was produced in the same way in this case, very fine-grained, as-deposited weld metal. I believe that part of the error in conversion tables arises from the variety of material conditions. Hardness is a deceptively simple test, but the result depends on a complex aggregate of physical and mechanical factors. If I must use Rockwell on a weld, I will choose smaller loads, such as the HRA scale.
HV with 0.5kg or 1kg load is pretty much the standard for weld evaluation (its now embedded in NACE and API standards, which recognize that HRC can underestimate hardness of small zones like the CGHAZ), but at these loads there is little risk of hitting individual carbides or so-called 'local hard zones'.
HRC is only suitable for larger regions like the weld zone or base metal. The selection of test method (= indentation size) is always a tradeoff. "Everyone is entitled to their own opinions, but they are not entitled to their own facts." RE: Conversion of Vickers Microhardness to Rockwell IRstuff (Aerospace) 19 Mar 19 20:19 This I believe will help close the gap between micro and macro hardness. The process is repeated on welds with different hardness levels to produce a facsimile of a calibration curve for a single material, over a limited range of hardness. The results of the parallel testing on the same material will produce a correspondence that is more valid than E140, for a narrow range of +/- around the result. I am suggesting the following protocol to establish a better correlation, for which I solicit your expert opinions:ġ) Produce a carbon steel bead-on-plate weld prepare a surface parallel to the plate surface to produce a block suitable for hardness testing.Ģ) Perform Vickers hardness in a local area (several points, so as to obtain a mean result), clean it up and perform Rockwell B or C at the same location. (I won't go into some other testing practices I consider shoddy.)Į140 tables are based on best-fit correlations between test methods that is based on a variety of product forms, production methods and heat treatments, so even with the best testing practices there is scatter.
I believe this is causing small but significant errors in our Rockwell numbers, threatening rejection of weld trials. They (like many other commercial labs, I suspect) will glibly convert using ASTM E140, ignoring the warning against this practice given in ASTM E92, Para. The problem: I'm having a disagreement with the way several reputed labs are doing conversion into other scales (Rockwell B/C). The specification imposes hardness limits on all weld zones. The current activity is assessment of carbon steel welds, where microindentation testing with Vickers (ASTM E384 0.5kg or 1kg indentation load) is the only sensible method.