Technological application of the martensitic transformation of some austenitic stainless steels

Abstract

The final heat treatment of austenitic stainless steels of types X 5 CrNi 18 9 (1.4301) and X 2 CrNi 18 10 (1.4306) normally is annealing at 1050°C and subsequent water quenching. The resulting structure is of a metastable fee-type. Plastic deformation, especially at low temperature, causes martensitic transformation of these metastable structure. The transformation is accompanied by a substantial flow stress increase. This strengthening mechanism should be used in prac tice, e.g. for saving weight /1,2/. The deformed structure consists of tetragonal a'-martensite, austenite and hep c-martensite. Whereas a 1 -martensite increases continuously with deformation, the content of c-martensitc reaches a maximum value at about 5% plastic strain at 77 K. The hep phase is only detectable by means of x-ray analysis whilst a'-martensite can be determined quantitatively by saturation magnetisation measurement. The flow stress increase in low temperature deformation of meta stable austenitic stainless steels is based on normal work-har dening by dislocation accumulation, in addition to a distinct amount of work-hardening due to martensitic transformation. Analysis of the work-hardening behavior in the range of stable deformation (T > M ) can be used to predict the amount of normal work-hardening when deformation is performed in the instable temperature regime. On the basis of figure 1 the corresponding flow curves can then be predicted /3/. Separation of the flow stress contributions according to the procedure described above enables the possible savings in weight to be predicted when using cryogenically stretched instable austenitic steels in comparison with stable grades deformed under the same conditions.

Microstructure, Residual Stresses, and Strain-Rate-Dependent Deformation and Fracture Behavior of AISI 304L Joints Brazed with NiCrSiB Filler Metals

“…Thus, the BCC-Fe formation in the DZ could be due to two possible microstructural causes. The first is the fact that AISI 304L steel is a metastable austenite [35,36], and ferrite formation may occur due to the depletion of austenite stabilizers, which go into steel grain boundaries, combining with chromium borides [15]. The second is the fact that the brazing seam is a heterogeneous construction that consists of different phases with different thermal and mechanical properties.…”

“…The second is the fact that the brazing seam is a heterogeneous construction that consists of different phases with different thermal and mechanical properties. These differences accumulate the stress state due to cooling after brazing, which may induce martensite formation [35][36][37]. Additional residual stress investigations for understanding this thesis are given below.…”

Influence of the Martensitic Transformation on the Fatigue Life of Austenitic Stainless Steels

“…Austenitic stainless steels are widely used in different sectors, such as architecture, automotive, chemical industry, domestic components, etc. For those applications where high strength is required, two main methods can be used to increase their mechanical resistance: one is age hardening; the other is via the transformation of austenite to martensite [1,2].…”