is there a maximum hardness level for 4140
is there a maximum hardness level for 4140
(OP)
hi every one i was told that 4140 should never be used past a hardness of 42 Rockwell max is this true? we have been using this matrial at 48-52roc for quite awhile and havent had a problem till reciently the pieces just shattered with not much force and this particular part could cause some major acident if it failed at the wrong time. any help or ideas are greatly appreciated.





RE: is there a maximum hardness level for 4140
RE: is there a maximum hardness level for 4140
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RE: is there a maximum hardness level for 4140
RE: is there a maximum hardness level for 4140
RE: is there a maximum hardness level for 4140
2. As metengr pointed out, tempering some alloy steels in certain temperature ranges can result in brittleness such as you describe. Depending on the metallurgical phenomenology that is causing your part to shatter, the problem could be due to either 'temper embrittlement' or 'blue brittleness'. 4140 alloy is supposedly more prone to the latter. Whatever the differences are, try staying out of the 450 -700° F tempering range. Impurities such as those already mentioned are a part of the problem. (To optimize toughness on one of our alloys we specify that the sum, P + Sn + Sb + As < [X-amount].)
3. For further details and rational on embrittlement causes and cures, if you have access, check out:
ASM Handbook, Vol 1, 10th Ed., pp 432, 698
Reed-Hill, Physical Metallurgy Principles, 4th Ed., p 705-6
SAE spec AMS-H-6875A, Heat Treatment of Steel.., Table 1A.
These references show a slighly higher austenitizing temperature than you are using, so you may want to consider bumping that up. It may have a secondary effect.
RE: is there a maximum hardness level for 4140
RE: is there a maximum hardness level for 4140
We had some Q&T'd straightener tooling initially in the 48-52 range that we lowered the hardness into the mid 40's due to pieces chipping and nearly hitting the operator. Sacrificed some wear but safety is more important.
Also the ASM Heat Treater's Guide recommends an austenitizing temperature of 1570F for hardening 4140 steel. As Bestwrench said, increasing the temp would be in order, at least to 1550F.