Blow holes held together with slag

An aerospace engineer once said that automobile steel was akin to blow holes held together with slag. While it is true that low carbon SAE 1018 or 1020 steel has relatively low yield strength and questionable purity, these steels are more than adequate for their intended application as millions of automobiles can attest. However, for strain gage based load cells, these steels just will not do the job.

A strain-gage-based load cell typically has a full scale output of 2-3 mv/v. In steel, this equates to a stress level of  30-45 KSI. Keeping a safe overload level and following a guideline of keeping working stresses below 50% of yield stress means the steel should have a yield of 100 KSI minimum. Two common steels, 4140 and 4340, exceed these requirements after heat treatment and are easily purchased in high purity by specifying electric furnace grade (E4140 and E4340).

For stainless steel, the same purity requirements are needed.  So, when purchasing the popular 17-4ph alloy be sure to specify vacuum arc remelt grade, not air furnace grade. If vacuum arc material is not available, specify 15-5ph stainless instead. 15-5 is always supplied in vacuum furnace grade.