China 42CrMo vs ASTM 4140/4142

[jasgeer]

We are in the process of sourcing machined parts in China and need to know whether the Chinese material specification GB/T3077 for their 42CrMo is equivalent to our ASTM/AISI 4140 and 4142 (we use both interchangeably). The obvious metallurgical difference is that 4140/4140 have trace percentages of P and S. The other elements (C, Mn, Si, Cr, Ni and Mo) are all similar but not exact. Assuming we get equivalent mechanical properties, can we expect the Chinese material to perform the same in a pressure-boundary application?
The Chinese are also proposing to make bolts out of the same material to replace our current use of ASTM 8740 or 8637 or 4037. Is this a good material substitute for bolts?
We are fortunate to have a manufacturing engineer that is fluent in Chinese, but we do not have a metallurgist on staff.
Are there questions we should ask the Chinese?
Thanks for the advice.
Jim

 

[TVP]

The chemical composition will be similar, but not exact. You will need to identify whether or not the following are important:

1. Fatigue strength
2. Fracture toughness

Sulfur and phosphorus are both quite detrimental to fatigue strength and fracture toughness. Other elements to consider are Al and V. These two control grain size, which has an effect on fracture and fatigue properties as well. Make sure that the grain size is equivalent to what you would be getting from the current supplier.

Other elements to consider are O & N-- again these influence the fatigue and fracture properties, with elevated levels contributing to much worse performance. Nonmetallic inclusions (sulfides, silicates, aluminates, globular oxides) are the last item to address. ASTM E 45 is one test method for determining inclusion content. The bulk composition may be the same, but inclusion levels can vary dramatically, resulting in much worse performance. You will need to work very closely with the vendor in order to address all of these issues. Good luck.