MATERIALS SCIENCE AND ENGINEERING

MICROSTRUCTURAL CHARACTERIZATION OF LOW-TEMPERATURE-CARBURIZED STAINLESS STEELS 13-8Mo AND AL6XN

A novel, low-temperature carburization technique has been established for surface hardening stainless steel in cooperation with Swagelok Company [1-4]. The process introduces extremely high carbon concentrations in the near-surface region of structural stainless steel alloys, surpassing the equilibrium concentration by several orders of magnitude at the process temperature of 470 °C, and even more so at room temperature. The colossal supersaturation is accompanied by a tremendous increase in surface hardness and corrosion resistance.

For the explanation of this remarkable result, a model has been created based on the concept of para-equilibrium. This means that the process temperature lies in a range where diffusion of substitutional solutes like iron, chromium, and nickel is extremely low, while carbon diffusion is still considerable. The striking discovery of extremely improved mechanical and electrochemical properties resulting from the colossal supersaturation with carbon suggested to apply the new technique to further types of steels – especially to non-austenitic stainless steels, such as 13-8Mo. Currently, mechanical and corrosion tests are conducted in order to prove the positive effect of the gas-phase carburization on the alloy properties.

13-8Mo (containing 13 mass% Cr, 8 mass% Ni, and about 2 mass% Mo) represents a special case of stainless steels, since it is ferritic/martensitic at room temperature – different from most other stainless steels, which are austenitic. Furthermore, it offers a much higher yield strength and ultimate tensile strength than 316-type austenitic stainless steel. The results obtained to date are very encouraging and may pave the way for a whole new field of surface treatment of body-centered cubic alloys.

AL6XN, which is highly alloyed with Ni (24 mass%), Cr (20.5 mass%), and Mo (6.5 mass%) is especially appreciated for its superior resistance against all different types of corrosion as well as in any kind of corrosive environment. Depending on the corrosive environment, the corrosion resistance is up to 15× better than that 316 stainless steel. Mechanical stress properties are at about the same level. However, ductility and toughness are of outstanding performance. Since AL6XN is not able to be hardened by conventional heat treatments, the proposed Swagelok process allows the use of an elegant, yet economical solution of surface hardening.

It is the goal of this investigation to characterize the microstructure of both 13-8Mo and AL6XN and to establish suitable processing parameters. This work is done by use of light-optical microscopy, SEM (scanning electron microscopy), TEM (transmission electron microscopy), AFM (atomic force microscopy), MFM (magnetic force microscopy), XPS (X-ray photoelectron spectroscopy or ESCA –electron spectroscopy for chemical analysis) and XRD (X-ray diffractometry) before and after treatment. These techniques help to analyze the composition in dependence of the depth and the presence and possible locations of second phases like carbides (Fig. 1, Fig. 2). Results of the mentioned experiments may lead to a better understanding of the hardening mechanism and allow to increase the case depth as well as optimize the whole process parameters for the examined alloys.

Fig. 1. Light-optical micrograph showing a cross section of low-temperature-carburized 13-8Mo steel.

Fig. 2. MFM (magnetic force microscopy) image showing a cross section of low-temperature-carburized 13-8Mo steel.