June 14, 2022
If my last posting prompted you to look at a copy of a material cert, you probably noticed that the cert has much more information on it than just the chemical analysis of the barstock or ingot. Hopefully you will never need to reference the heat lot code, or the specific mill that poured the ingot. Those details only come into play when endemic failures are being sorted out and fingers are being pointed. Many of the acronyms you encounter, like VAR for Vacuum Arc Re-melt, are of interest for critical service applications. Compliance to certain standards is an indication of the care taken in making and processing the material. It will not hurt you to use “aircraft quality” material in a commercial part. It would be a serious crime to use non-compliant heat lots for an aircraft component, however. If you make parts on a rush basis, you may be asked to approve material that lacks the required “pedigree” on the material certification. This is where reading the fine print becomes important, since some deficiencies in pedigree can be remedied after the fact with additional testing or processing. Material cleanliness, for example, can be checked on a sample from the proposed material. Grain size can be verified. Barstock can be ultrasonically tested — even before rough turning. Ever wonder why “aircraft bars” come in random length? It is because the mill did an ultrasonic test and lopped off anything that looked the slightest bit suspicious. For critical uses, some contracts will insist upon these tests — regardless of the cert claims. Crack checking after rough machining or thermal processing may allow “commercial-grade” materials to be accepted. Some companies have established policies for material substitutions. If the “right stuff” is not available in the size and grade [chemistry] listed on the drawing, you may find moving “up” in either will resolve the problem. Material cost is typically only a small portion of part cost. It is better to pay a few cents a pound more up-front than to risk an unacceptable finished part.