The Effect of Strain on Stainless Steel Surface Finish

The bright surface finish of exterior automotive moldings made from stainless steel can become hazed and reflections distorted as a result of forming done during the manufacturing processes. Bright moldings are frequently used to give styling differentiation accents to vehicle exteriors. Stainless steel provides cost effective differentiation with a material that is durable and relatively easy to form to shapes desired by the stylist. Because of the desirable attributes of stainless steel, an understanding of the threshold of unacceptable surface appearance is necessary to maximize showroom appeal and avoid customer complaints that result in warranty claims. This paper quantifies the effect that manufacturing strain and strain rate have on the surface finish of 436M2 stainless steel. Controlled experiments were conducted on production grade stainless steel strips subjected to a variety of strain and strain rates typical of manufacturing processes. Resultant surface finishes were evaluated through both objective and subjective analyses and recommendations are made to the maximum strain and strain rates permitted to avoid objectionable changes to the surface finish. A method is shown to predict the tensile strain induced during stretch bending of exterior moldings. When combined with strain limits for acceptable surface finish identified earlier, a graph of the “design range” of acceptable stretch bent molding geometries is developed. This predictive tool can be used early in the design process to guide styling and molding concepts, ensuring good surface finish quality long before the first part is produced.