Measurement of deformation hardening of stainless steel sheet : the correlation between process parameters and material properties

Stainless steel is the general name for different steels used primarily for their resistance to corrosion. One of the major advantages of the stainless steels, and the austenitic stainless steels in particular, is their ability to be fabricated by all the standard fabrication techniques. Austenitic stainless steels also have very high ductility and are in fact capable of being very heavily cold formed, despite their high strengths and high work hardening rate. Few other metals are capable of achieving this degree of deformation without splitting. Outokumpu Stainless has developed stainless steel HyTens 1000, which is a metastable austenitic stainless steel. It is deformation hardened by cold rolling up to an ultimate tensile strength of 1000 MPa. When forming metastable austenitic steel, martensitic transformation occurs. Since martensite is harder than austenite the material is deformation hardened. The aim in this report is to find a suitable method to measure the strength in a material that has been subjected to deformation hardening. The material used was HyTens 1000. The methods that are used to estimate the strength in a material have been hardness measurement, magnetic induction - eXacto, X-ray diffraction, tensile testing, LDH – Limiting Dome Height, and LDH- and S-beam strain measurement. • Hardness testing can be conducted on deformed metal sheet but it is a destructive test. • The eXacto is a non-destructive test method to measure the martensite content. It is fast and accurate when used at known thicknesses. This method is by far the easiest of the different methods used. • The x-ray diffraction is time consuming and dependent on the operator. Metal sheet from formed components cannot be tested since the sample has to be flat. • By conducting a tensile test one can get information about the Rm and Rp values of a material. But the samples that will be tested have to be standardized. It is often impossible to get a standardized sample with uniform deformation from an already deformed metal sheet. • The LDH method demands a non-deformed metal sheet of approximately 60*60 cm and is therefore not a suitable method to measure material properties in the industry. • The strain measurement method is non-destructive, but it is not always possible to see the correlation between strain and martensite content when changing the process parameters. There are still some results that can be made to get a complete test matrix. For example there are more hardness measurements that can be done, as well as more x-ray diffraction tests. By doing more tests on the correlation between Rm and eXacto more precise curves could be established for different thicknesses. By doing these tests on different thicknesses one would be able to establish how sensible the eXacto is to the thickness of the metal sheet. The ideal instrument to measure the martensite content would be as easy to use as the eXacto, but with a devise that measures the metal sheet thickness at the same time as measuring the magnetism and then converts the results to Rm. The question remains if the market is willing to develop this kind of instrument, how large is the interest in measuring Rm on formed details of deformation hardened stainless steels?Validerat; 20101217 (root