Elif Temiz, 030070195, 10th Week Definitons

1-Precipitation-Hardening Stainless Steels

New Definition (Material)

The term hardness in its largest sense evokes four different properties. One sense of the word is the resistance of steel to penetration. The value of this concept of hardness is useful for selecting materials suitable for valves and flanges in vessels for tritium reprocessing. Another definition is that of the elasticity limit of steel. This definition applies to metal assemblies supporting for instance heavy process equipment and piping in the nuclear industry and the metal flanges with valves in tritium processing, that are also important. Corrosion tensile and fatigue resistance tests are interpreted as tests of hardness. These are useful for selecting nuclear facility materials. The precipitation hardening stainless steels are subdivided into, semi-austenitic and austenitic types. The martensitic types are generally supplied in the martensitic condition and precipitation hardening is achieved by a simple aging treatment. The semi-austenitic types are supplied in the austenitic condition and austenite must be transformed to the martensite phase by heat treatment before precipitation hardening. In the austenitic types, austenite precipitation hardens the phase directly. The precipitation hardening process involves the formation of very fine intermetallic carbides which impede dislocation during deformation, giving rise to higher strength. Precipitation hardening generally results in a slight reduction of corrosion resistance and an increase in susceptibility to hydrogen embrittlement.

In order to obtain a better understanding of the metallurgical changes underlying these heat treatments, it is instructive to refer to the regions of stability of the various phases as a function of carbon concentration. The first step in the heat treatment sequence of precipitation hardening stainless steels is annealing at temperature in the single-phase austenite region. The subsequent step is stabilizing austenite during a relatively rapid cooling. Precipitation hardening is achieved by reheating the austenite to high temperatures at which the line intermetallic compounds are precipitated. Some minor reversion of the martensite to austenite may also occur. The martensitic precipitation hardening steels are obtained by adjusting the composition. The chromium concentration is in the range of 10-18%, nickel 5-12.5% and carbon plus nitrogen 0.03-0.17%.

Precipitation hardening stainless steels, referred to as semi-austenitic, require an intermediate step to transform the metastable austenite to martensite before precipitation hardening. This can be achieved by tempering at about 750°C to reduce the carbon and chromium concentration of the austenite by the precipitation of carbide. Reduction of dissolved carbon and chromium and cold working can be used to accelerate the transformation of austenite to martensite. Martensite precipitation to high strength levels introduces susceptibility to hydrogen embrittlement.

 Precipitation hardening stainless steel surfaces by high pressure is also of interest. Structure can be changed by polygonization as shown in Fig. 5.10 realized with 316 stainless steel exposed to a high force of 10² N mm^-2. Surface hardening concerns low thicknesses and, austenite is transformed to the martensite phase (Fig. 5.11). Hardening and martensite concentration depend on the shock pressure (Figs 5.12 and 5.13). Laser cladding is also an innovative and attractive manufacturing route for producing a thin hardened surface with martensite (Maraging and 17-4 PH stainless steels.) Directly after manufacturing, these steels (Fig. 5.14) are sound and metallurgically bonded to the substrate. Their mechanical features are fairly good: harness: 800 HV, Young’s modulus: 210 GPA, yield strength: 1200MPa. Applications are special valves and tight flanges in tritium processes.

(Gilbert Bellanger, Corrosion Induced By Low-energy Radionuclides: Modeling Of Tritium And Its Radiolytic And Decay Products Formed In Nuclear Installations, pp.83,85)


There is no previous definition.