Machining Allowances on Hot Rolled Bar

[Carburize]

Can anybody contribute any direct experience on the suggested machining allowance (stock removal) for hot rolled steel bars. The standard allowance is 2% of diameter per side, does this work consistently to remove any metallurgical imperfections in the surface?

 

[Maui]

It depends upon the grade of steel in question, the processing history, and the specifications to which it was produced. In special cases the requirements of the application may also impact the minimum removal. According to ASTM A-600 92a, table 5 the maximum decarburization limits for a hot rolled round high speed or tool steel bar between 2 and 3 inches in diameter is 0.050 inches per side. In this table it is also noted that the recommended minimum allowance for machining prior to heat treatment is 25% greater than the maximum decarburization allowed. So for a 2.500" diameter hot rolled high speed steel bar, the minimum machining removal on the diameter would be 0.125". This represents a removal of 5%. A 2% removal in this case would most likely result in some amount of remnant decarb on the surface of the heat treated bar, which could adversly impact the performance of the tool made from it.


Maui

 

[TVP]

As maui mentioned, 2% is not necessarily a conservative value. For resulfurized grades under 2", the machining allowance should be ~ 3.4% for centerless turned or ground HR bars, and ~ 2.4% for machined bars. Over 2", the allowances should be 2.4% for either condition. Non-resulfurized grades under 2" should be 2.6% and 1.6%, respectively, and 1.6% for bars over 2". This information is from Ispat Inland Bar Products. MACSTEEL uses the generic 3% for resulfurized and 2% for non-resulfurized grades, but their round billet is not as susceptible to decarb as the corners of a square billet is. Ascometal in France is the only other round billet producer of which I know, but I don't have any comparison data for them.

 

[BobUXL]

You may want to look at ATSM A484 which will list the max defect allowable in hot rolled steels. Cold drawn materials typically use a 1% diameter and the max defect per side. 2% seems reasonable, even for decarb on high carbon grades. But with this said, good quality mills will try to minimize the defect depth (don't forget, they use the same hot rod to produce other products and they wangt to maximize yields). Bottom line, know your producer.

 

[TVP]

ASTM A 484 is a standard specification for stainless steels. The machining allowance is smaller for stainless steels than carbon or alloy steels.

 

[BobUXL]

The ASTM spec is indeed for stainless and there are other ASTM specs that cover alloy/tool steel materials as well. I am just aware of A484. However, this is a good guideline and if you realize that high carbon alloys will decarb/scale more than many stainless, you can add another percent for good measure. I believe that a high carbon tool steel alloys .063" for a 6" rd for decarb.

Sorry I cannot be more definitive for other alloys.