Some Strengthening Methods for Austenitic Stainless Steels

Austenitic stainless steels possessing good corrosion resistance have recently found growing applications as a constructional material. In this instance, increasing strength properties, which are typically quite low, is of great interest. Due to the low stacking fault energy, strain hardening of alloyed austenite is efficient for increasing tensile strength without impairing ductility seriously. In addition, certain grades are unstable, so that cold working creates strain‐induced martensite that enhances strengthening. Grain size refinement to micrometer scale or even finer can also increase the yield strength, still providing good ductility. In the present paper dislocation and phase transformation strengthening and thereby properties achievable in temper rolled austenitic stainless steels are discussed. Strengthening by the reversion annealing is also described and excellent results achievable are shown. Finally, the effect of bake hardening through the static strain ageing is presented. Long‐term research work in various projects indicates that the current knowledge of strengthening of austenitic stainless steels is close to the industrial utilisation.     

氢致奥氏体不锈钢晶格畸变

对304L和316L奥氏体不锈钢试样在充氢后和充氢同时进行X射线衍射分析,观察到在充氢过程中存在奥氏体晶格膨胀-收缩-膨胀的现象;在充氢后时效一段时间的情况下,存在奥氏体晶格收缩-膨胀-收缩的现象,并初步讨论了可能的原因。     

Defining the Post-Machined Sub-surface in Austenitic Stainless Steels

Metallurgical and Materials Transactions A - Austenitic stainless steels grades, with differences in chemistry, stacking fault energy, and thermal conductivity, were subjected to vertical milling....     

Sheet Formability and Performance of Metastable Austenitic Stainless Steels

The sheet formability of AISI Types 301, 304, and 305 stainless steels, ranked in order of increasing stability to strain‐induced martensite formation, was evaluated as a function of temperature between 15 and 60 °C. Forming limits spanning deep drawing, plane strain, and biaxial stretching strain states were determined by circle grid analysis of sheet specimens subjected to punch‐stretch testing at a constant punch displacement rate. Amounts of strain‐induced martensite were measured as a function of strain by magnetic measurement. Formability varied widely depending on test temperature and austenite stability, a result of temperature‐ and strain‐dependent formation of martensite that in some conditions was beneficial and in some conditions was detrimental to formability. These results are presented and discussed in detail.     

Mechanical Behaviour of Aged High Nitrogen Austenitic Stainless Steels

The present investigation is aimed at studying the effect of ageing on mechanical behaviour of two high nitrogen austenitic stainless steels as these steels are exposed to high temperatures in their applications. Two steels with nitrogen contents greater than 0.6% were given solution treatment and were aged at 500–900°C for 1–100 h. It was found that the steels exhibit superior mechanical properties in solution treated condition while ageing has a deleterious effect on properties due to weak interfaces of depleted austenite matrix and Cr₂N lamellae. The plastic flow behaviour can be modeled using modified Ludwik equation in these steels.     

Characterization of Austenitic Stainless Steels Deformed at Elevated Temperature

Highly alloyed austenitic stainless steels are promising candidates to replace more expensive nickel-based alloys within the energy-producing industry. The present study investigates the deformation mechanisms by microstructural characterization, mechanical properties and stress–strain response of three commercial austenitic stainless steels and two commercial nickel-based alloys using uniaxial tensile tests at elevated temperatures from 673 K (400 \(^{\circ }\)C) up to 973 K (700 \(^{\circ }\)C). The materials showed different ductility at elevated temperatures which increased with increasing nickel content. The dominating deformation mechanism was planar dislocation-driven deformation at elevated temperature. Deformation twinning was also a noticeable active deformation mechanism in the heat-resistant austenitic alloys during tensile deformation at elevated temperatures up to 973 K (700 \(^{\circ }\)C).     

High Strength Stainless Austenitic CrMnCN Steels – Part I: Alloy Design and Properties

Generally the strength of stainless austenitic steels does not live up to their good corrosion resistance. Solid solution hardening by interstitial elements is a means of raising the strength, but is used only moderately because of poor weldability, which, however, is not required in various applications. The solubility of nitrogen is high in stainless austenite of steels with 18 mass% of Cr and Mn each, but low in the melt. Carbon reveals the opposite behaviour. Instead of producing high nitrogen steels by pressure metallurgy, about 1 mass% of C+N is dissolved in the melt at ambient pressure. The new cost‐effective C+N steel reaches a yield strength of 600 MPa, a true fracture strength above 2500 MPa and an elongation above 70 %. Conduction electron spin resonance revealed a high concentration of free electrons. Thus, the ductile metallic character of the C+N steel is enhanced, explaining the high product of strength times toughness. The high interstitial content requires rapid quenching to avoid an embrittling precipitation and respective intercrystalline corrosion.     

Behaviour Model of Austenitic Stainless Steels for Automotive Structural Parts

One of the most important preoccupation of car manufacturers is to reduce emissions and hence to reduce weight of cars. One of the outstanding materials able to reduce weight while at the same time keeping the same crash absorption and hence safety, is austenitic steel. Austenitic stainless steels are used in crash relevant parts of cars. Moreover, designers can use their very good corrosion resistance and their well known surface aspect for structural visible parts like wheels, cross members, roof panels or tailgates. In this paper, stainless steels for automotive use are presented in detail. First, their chemical composition and tensile properties are explained. Then, a model for forming and crash behaviour is described. Using this model, stainless steels can be engineered into automotive parts and thus stainless steel can be considered as a workable and predictable material for the automotive industry.     

Sensitization Behaviour of Manganese‐Alloyed Austenitic Stainless Steels

Manganese is an essential alloying element in advanced austenitic stainless steels with specific properties such as high resistance to harsh corrosive environments, high strength or low material costs. These materials are often used for welded constructions which have to be highly corrosion resistant. Hence it has to be ensured that the heat input during welding does not initiate the precipitation of chromium carbide resulting in a susceptibility to intergranular corrosion. This leads to the question whether the sensitization behaviour of manganese‐alloyed austenitic stainless steels is comparable to that of the well‐known conventional chromium nickel austenites. In the present work the effect of heat‐input on the susceptibility of the CrNi‐steel 1.4301 and the CrNiMn‐steel 1.4376 to intergranular corrosion (IGC) was considered. Investigations were carried out by corrosion testing in the so‐called Strauss‐Test to elucidate the effect of the annealing temperatures on the microstructure. Furthermore, the influence of heat treatment on the mechanical properties was evaluated by tensile testing. As a result, it could be demonstrated that manganese‐alloyed austenitic stainless steels like grade 1.4376 exhibit a sensitization behaviour very similar to the conventional austenitic steel grades. The same kinds of tests on intergranular corrosion resistance can be applied for both types of materials.     

Fatigue and Structural Changes of High Interstitial Stainless Austenitic Steels

Steels with 18 to 19 mass% Cr and Mn each were studied in the as‐cast condition containing 0.85 mass% C + N and in the elektro‐slag‐remelted and hot worked condition containing 0.96 mass% C + N after final solution annealing. The latter was also tested after 20% prestraining. The results of tensile tests were compared to those of rotating bending and push/pull loading. The higher C + N content raised the 0.2% proof strength to about 600 MPa of which 70% were retained as fatigue limit of rotating bending at 10⁷ cycles and a failure probability of 50%. Prestraining further improved this limit but lowered it in relation to the proof strength. The structural components of cold work hardening under unidirectional loading and cyclic loading were similar (planar slip, dislocation, twins and ε‐martensite) except for precipitates in the latter. Nitrides appeared in the austenite and carbides in the ε‐plates.     

Research Progress and Development Tendency of Nitrogen-alloyed Austenitic Stainless Steels

Research progress on nitrogen-alloyed austenitic stainless steels was expounded through the development of steel grades.In addition,hot topics in the research of nitrogen-alloyed austenitic stainless steels were discussed,including the solubility of nitrogen,brittle-ductile transition,and welding.On this basis,it was proposed that the future development tendency of nitrogen-alloyed austenitic stainless steels lied in the three fields of high-performance steels,resource-saving steels,and biologically friendly steels.The problems encountered during the research of nitrogen-alloyed austenitic stainless steels were discussed.     

Metallurgical Studies of Austenitic Stainless Steel 304 under Warm Deep Drawing

Austenitic stainless steel 304 was deep drawn with different blank diameters under warm conditions using 20 t hydraulic press. A number of deep drawing experiments both at room temperature and at 150 ℃ were conducted to study the metallography. Also, tensile test experiments were conducted on a universal testing machine up to 700 ℃ and the broken specimens were used to study the fractography of the material using scanning electron microscopy in various regions. The microstructure changes were observed at limiting draw ratio （LDR） when the cup is drawn at different temperatures. In austenitic stainless steel, martensite formation takes place that is not only affected by temperature, hut also influenced by the rate at which the material is deformed. In austenitic stainless steel 304, dynamic strain regime appears above 300 ℃ and it decreases the formability of material due to brittle fracture as studied in its fractography. From the metallographic studies, the maximum LDR of the material is observed at 150 ℃ before dynamic strain regime. It is also observed that at 150 ℃, grains are coarse in the drawn cups at LDR.     

Hydrogen Induced Slow Crack Growth in Stable Austenitic Stainless Steels

The behavior of hydrogen induced slow crack growth in type 310 and type 16-20-10 stable austenitic stainless steels along with type 321 unstable austenitic stainless steel were investigated. It was found that slow crack growth could occur in all three types of stainless steels, and the threshold values wereK _H/K_c = 0.55, 0.7, and 0.78 for type 321, 310, and 16-20-10 stainless steel respectively, when charged under load. Slow crack growth could also occur if the precharged specimens were tested under constant load in air. No slow crack growth occurred in the precharged and then out-gassed specimens. This indicates that delayed cracking in stable austenitic stainless steels is induced by hydrogen. Since there is no hydrogen induced α’ martensite in type 310 and 16-20-10 stainless steel, the existence of a’ martensite is not necessary for the occurrence of slow crack growth in the austenitic stainless steels, although it can facilitate slow crack growth. The mode of hydrogen induced delayed fracture in either the stable or unstable austenitic stainless steel is correlated with theK, value; the fracture surface is changed from ductile to brittle asK ₁ is decreased.     

High Strength Stainless Austenitic CrMnCN Steels ‐ Part II: Structural Changes by Repeated Impacts

Phase transformations and changes in the structure caused by impact loading of steel Cr18Mn18CN alloyed with carbon+nitrogen are studied in comparison with Hadfield steel using X‐ray diffraction, Mössbauer spectroscopy and TEM. It is shown that the surface layer of all the studied steels impacted by mineral particles of greywacke remains austenitic, although their magnetic structure within a depth of about 10 μm is similar to that of martensite. TEM studies reveal a mixture of amorphous, nanocrystalline and thin‐twinned fcc crystal structures. The suggestion is made that the atomic configuration at the twin boundaries is similar to that in the bcc lattice and induces the high‐spin state of the iron atoms in the thin twins. At the same time, the amorphous structure of the surface layer can be also a source of ferromagnetism like this occurs in rapidly quenched FeSiB ribbons. The extremely high wear resistance of the newly developed CrMnCN steels and of Hadfield steel seems to be related to the impact‐induced surface structure in its amorphous + nanocrystalline + thin‐twinned austenitic states.