Toughness of Stainless Steel - China Stainless Steel Tube Manufacturer

Toughness of Stainless Steel

The effects of temperature, composition, and weld-process variations on the fracture toughness behavior for Types 308 and 16-8-2 stainless steel welds were examined using the multiple-specimen J-resistance-curve procedure. Fracture characteristics were found to be dependent on temperature and weld process, but not on filler material. Gas-tungstenarc (GTA) welds exhibited the highest fracture toughness, a shielded-metal-arc (SMA) weld exhibited an intermediate toughness, and submerged-arc (SA) welds yielded the lowest toughness. Minimum expected fracture properties were defined from lower bound fracture toughness and tearing modulus values generated here and in previous studies.

Fractographic examination revealed that microvoid coalescence was the operative fracture mechanism for all welds. Second-phase particles of manganese silicide were found to be detrimental to ductile fracture behavior because they separated from the matrix during the initial stages of plastic straining. In SA welds, the high density of inclusions resulting from silicon pickup from the flux promoted premature dimple rupture. The weld produced by the SMA process contained substantially less manganese silicide, while GTA welds contained no silicide inclusions. Delta ferrite particles, present in all welds, were substantially more resistant to local failure than the silicide phase. In welds containing little or no manganese silicide, delta ferrite particles initiated microvoid coalescence, but only after extensive plastic straining.

Among the materials that withstand corrosion, stainless steel shows an excellent resistance in a large number of atmospheres, due to a phenomenon known as passivity. Stainless steel is protected from its environment by the formation of a very thin passive film or passive layer. It is strongly bonded to the surface, which prevents further direct contact between the metal and its more or less aggressive environment. In Stainless Steel, the passive film also has the advantage, compared. Chemical or mechanical damage to the passive film will heal or repassivate in oxidising environments.

Physical properties and mechanical properties (toughness, strength and ductility), ease of fabrication (particularly ease of forming) excellent fatigue resistance and energy absorption capability are some of the properties of Stainless Steel which enable the specific requirements of structural components to be met. The main advantages of Stainless Steel as a structural material are the exceptional combination of relationships that are developed in paragraphs 2 and 3.

2. Fracture Toughness (K) versus Strength (σ)

While strength is the controlling property if a component must withstand a specific load, toughness is the limiting property if a component must be capable of absorbing a specific quantity of mechanical energy without fracturing. In engineering structures, strength often must be combined with toughness, which indicates the amount of energy absorbed during the deformation and fracture. Austenitic and duplex Stainless Steel (Fe-Cr-Ni (Mo) alloys) and ceramics are compared in table 1. With the exception of austenitic and duplex Stainless Steel, most of the engineering materials, with high strength range go through a transition from ductile behaviour at room temperature to brittle at low temperatures. Thus to prevent brittle, i.e. catastrophic, failure, the service temperature of the structural component must be higher than the material’s ductile to brittle transition temperature. With austenitic and duplex Stainless Steels, the fracture toughness is independent of the temperature in the range of -200° C to 50° C.

Material Properties for Lightweight Structural Design The specific types of Stainless Steel under consideration in this application belong to two families according to their alloying element composition, which determines their metallurgical structure as well as mechanical properties. These two families are:

a) Duplex austenitic-ferritic stainless steel

The most commonly used duplex grade is 0.02% C – 22% Cr – 5.5% Ni – 3% Mo – 0.15% N alloy, whose standard European designation is X2CrNiMoN22-5–3 / 1.4462.

b) Austenitic stainless steel

These steel have chromium (18 to 30 per cent) and nickel (6 to 20 per cent) as the major alloying elements. The austenitic phase is stabilised by the presence of a sufficient amount of nickel. The principal characteristics are the ductile austenitic condition, rapid hardenability by cold working and excellent corrosion resistance. One of the most commonly used grade for structural applications is the 0.02% C – 17.5% Cr – 7% Ni – 0.15% N alloy, whose standard European designation is X2CrNiN 18-7/1.4318.

Young’s Modulus (E) versus Density (ρ)

The stress-strain relationship (in its linear part) is usually described by Young’s modulus: E = σ/ε, where σ is the “true stress” and ε is the “true strain”. Specific stiffness E/ρ is a reliable indicator of material performance in bending. A simple comparison of specific stiffness gives a good indication of stiffness resistance of different materials. As it can be seen in table 2, the specific stiffness of Stainless Steel is very similar to that of aluminium alloy and the HSLA steel, which means that the three materials can all be considered as “light materials”.