Difference between 4140 and 4140H steel

An "H" suffix can be added to any designation to denote hardenability is a major requirement. The chemical requirements are loosened but hardness values defined for various distances on a Jominy tes^[2

SAE steel grades Wikipedia.

They are the same alloy.

The H band composition provides the melt shop with a somewhat wider chemical range to assure that they have the degrees of freedom needed to assure that the material will harden within the bands as provided by astm a 304.

Most US mills melt to the tighter chemistry limits but dual certify to the standard and H band hardenability because their computer models and control technology enable them to hit their aims consistently.

Back in the days when we were dumping in ferroalloys with shovels and wheelbarrows, we needed more room than the standard spec to assure we got the hardenability because of yield loss, weight discrepancies etc.

In old days, commercial aspect could be if you order to h band, they warrant to it; if you order standard chem they will not warrant hardenability performance. But control technology is so good these days that the chances of hitting the chem and missing the hardenability are pretty slim.

Milo