Stainless Steel balls used in airbags

For over 20 years Thomson has supplied a unique stainless steel type 431 ball to the automotive industry to be used in the mechanical air bag sensor.  What makes this so unique is the material and tolerance required.  The ball had to be a perfect fit inside a tube to activate the air bag upon collision.  It had to expand and contract with a wide range of temperatures you would expect anywhere in the world where the cars were sold. The tube had already been selected  by the airbag manufacturer and Thomson had to come up with a material that had the correct coefficient of thermal expansion to expand and contract while maintaining the same clearance with the tube.  Thus stainless steel type 431 was selected.  No one had ever made a ball out of 431 stainless steel,  let alone achieved the close tolerances required on this soft  material.   This  ball required a spherical tolerance of 10 millions max , a size variation of +/- 10 millions max , surface finish of under Ra 1.  This wouldn’t be that difficult for a hard material like 52100 chrome or a stainless steel 440C which have Rockwell C hardness in the 60’s.  The  7/16 stainless steel 431 only has hardness in the Rockwell C 40 range.  The Thomson team was able to perfect the ball in less than 6 months.  Today the mechanical system has been mostly replaced with a   non-mechanical system. Of note- this special ball never had a field failure in over 20 years of use.