A. Di Schino

Effects of martensite formation and austenite reversion on grain refining of AISI 304 stainless steel

The austenite–martensite transformation followed by annealing for austenite reversion in AISI 304 stainless steel has been investigated in order to study the effect of this thermo-mechanical process on grain refinement. In particular the effect of cold reduction, annealing temperature and annealing times have been analysed. After getting ultrafine grains the effect of the grain size on the hardness and on the tensile properties has been evaluated, showing a Petch-Hall dependency in the fully analysed range (down to 0.8 μm grain size).

Development of ultra fine grain structure by martensitic reversion in stainless steel

Austenitic stainless steels have good corrosion resistance and good formability but they have also relative low yield strength. It is well known that the mechanical properties of austenitic stainless steels are very sensible to the chemical composition (which can induce hardening by both substitutional and interstitial solid solution) and to microstructural features (such as grain size and δ-ferrite content). Recently there have been commercial developments to exploit the effect of these variables in stainless steel taking advantage of changes in the chemical composition induced by nitrogen addition [1, 2]. Another effective way to increase yield strength without impairing good ductility is grain refining. Although this approach has induced the development of ultrafine grain carbon steels (e.g. [3]), no attempts have been still reported on this approach for austenitic stainless steels. In fact, austenitic stainless steels do not undergo phase transformation at typical annealing temperatures and then the only way to refine the grain is recrystallization after cold rolling. However, the strengthening by grain refining is limited, due to the high recrystallization temperature of this stainless steel grade. For instance, the recrystallization temperature of the AISI 301 steel is above 900 ◦C and the minimum grain size obtained is in the range 10–30 μm [4]. In austenitic stainless steels, plastic deformation of austenite creates the proper defect structure which acts as embryo for martensite deformation: the successive reversion of deformation-induced martensite (α′) enables a marked grain refining [5, 6]. In this letter the production of an ultra fine microstructure in an AISI 301 stainless steel by martensitic reversion is reported. The chemical composition of the steel used is shown in Table I. The procedure used to refine the grain is the following (see Fig. 1): • Metastable γ is almost entirely transformed to α′ by heavy cold rolling: in fact the retained γ cannot be refined during the subsequent annealing. • α′ reverts to recrystallized austenite γR during annealing at low temperature.

Grain refinement strengthening of a micro-crystalline high nitrogen austenitic stainless steel

Abstract Austenitic stainless steels have been indispensable for the progress of technology during the last 80 years. Due to the cost of nickel and to the prospective of allergic reactions caused by this element, more and more laboratories and industries are trying to develop a new class of austenitic stainless steels with a low nickel content. In order to maintain the austenitic microstructure, nickel reduction is balanced with nitrogen addition. Nitrogen addition to austenitic stainless steels is also very effective for improving yield strength and corrosion resistance without reducing ductility and toughness. In order to further increase the strength, it is possible to combine the effect of nitrogen addition and grain refining. The purpose of this study is to examine the relationship between microstructures and mechanical properties of a high nitrogen stainless steel (containing about 1% Ni and 0.37% N) with an ultrafine grained structure. In particular, the Hall–Petch dependency is found to apply over the range of grain size from 40 to about 2.5 μm.

Effects of the grain size on the corrosion behavior of refined AISI 304 austenitic stainless steels

Although there have been many studies on fine grained ferritic steels, only a few research reports are available on refined austenitic stainless steels and, in particular, on the influence of the grain size on the corrosion resistance of this class of material [1, 2]. The grain size of ferritic steels can be easily induced by phase transformation, but in austenitic alloys, following the absence of a phase transformation, the grain diameter is usually controlled by recrystallization after cold working [3]. This method is mainly affected by the working temperature, amount of deformation and recrystallization temperature. Recrystallization after hot rolling is reported to have the effect of grain refining [4] but this method seems to be limited. In a previous paper [5] we examined the effect of subzero working on the grain refining of austenitic stainless steels. In particular, ultrafine grained AISI 304 stainless steel of ca. 1 μm average grain size was obtained by applying the reverse transformation of martensite to austenite on subzeroworked steel annealed at low temperatures. Up to now, the corrosion behavior of such ultrafinegrained austenitic stainless steels has not been reported. This paper deals with the corrosion behavior, especially general corrosion (GC), intergranular corrosion (IGC) and pitting corrosion (PC) of ultrafine-grained AISI 304 stainless steel. Results are compared with those of similar measurements on standard AISI 304 steel. The chemical composition of the AISI 304 stainless steel, obtained from a commercial batch, is shown in Table I. After subzero working down to 90% thickness reduction, the material was subjected to the following four heat treatments in order to obtain different microstructures: annealing at 800 ◦C for 160 s and 900 s (specimens A and B respectively) and at 1000 ◦C for 10 s and 600 s (specimens C and D respectively). The grain sizes corresponding to the above specimens, as measured by automatic image analyzer, are shown in Table II. The typical microstructures of the 1 μm and 50μm specimens are shown in Fig. 1. Tensile properties of the specimens are shown in Fig. 2. Ultimate tensile stress and 0.2% yield stress increase with decreasing grain size, according to the Hall Petch relation [6]. Steel materials were machined into corrosion test specimens of 15 × 15 × 1 mm. The specimen surface was polished by using increasingly finer abrasive papers, starting with a 300 grit paper and finishing up with

Effects of grain size on the properties of a low nickel austenitic stainless steel

The effect of the grain size (varying in the range of 2.5–50 μm) on the mechanical properties and on the wear and corrosion resistance of a low nickel austenitic stainless steel is reviewed. In particular, the austenite-martensite transformation followed by annealing for martensite reversion in high nitrogen stainless steel is investigated. In order to study the effect of this thermo-mechanical process on grain refinement, the effect of cold reduction, annealing temperature and annealing times were analysed. After obtaining ultrafine grains, the effect of the grain size on the hardness and the tensile properties was evaluated and showed a Petch-Hall dependency in the fully analysed range (down to a 2.5 μm grain size).The fatigue behaviour of the steel is studied as a function of the grain size showing a poor influence of grain refining on the fatigue resistance. An increase of both the wear resistance and of the localized corrosion resistance with grain refining is also detected. Results are compared to those of similar measurements on a standard AISI 304 steel.

Development of high nitrogen, low nickel, 18%Cr austenitic stainless steels

Two high nitrogen stainless steels are studied through metallographic, mechanical and corrosionistic tests and the results are compared with those shown by a standard AISI 304. These high nitrogen steels show a significantly higher mechanical strength than usual AISI 304 while their corrosion resistance lie among that of standard austenitic and that of standard ferritic stainless steels.

Solidification mode and residual ferrite in low-Ni austenitic stainless steels

The solidification modes of two new classes of austenitic stainless steels with a low content of Ni are shown. Their chemical composition is similar to that of the standard AISI 304 and AISI 316, except for the content of nickel, manganese and nitrogen. It is found that standard formulas for predicting the residual ferrite can be fairly well used in the prediction of the solidification mode while they do not work in predicting the residual ferrite content. In particular, it is found that ferrite is the first phase to solidify for values of the equivalent ratio (calculated according to the formulas developed by Hammar and Svensson) greater than 1.50, otherwise austenite is the first phase to solidify. A new set of equations for predicting the residual δ-ferrite in these new classes of materials is determined via multivariable linear regression. The influence of the steel solidification mode on the material structural transformations during heat treatment is also shown.

Effect of grain size on the corrosion resistance of a high nitrogen-low nickel austenitic stainless steel

Nitrogen alloyed austenitic stainless steels exhibit attractive properties such as high levels of strength and ductility, good corrosion resistance and reduced tendency of grain boundary sensitization [1]. The high austenitic potential of nitrogen allows the nickel content in steel to be reduced, offering additional advantages such as cost saving. The production of these low nickel steels is made possible by the addition of manganese that increases the N solubility in the melt and decreases the tendency of Cr2N formation [2]. Although there have been many studies on finely grained ferritic steels (e.g. [3]), only a few research reports are available on refined austenitic stainless steels. The grain size of ferritic steels can be easily refined by phase transformation, but in austenitic alloys, due to the absence of phase transformation, the grain diameter is usually controlled by recrystallization after cold working [4]. In the last case the behavior of the material is affected mainly by the working temperature, working ratio and recrystallization temperature. Recrystallization after hot rolling is reported to have the effect of grain refining [5] but this method seems to be limited. In previous papers [6, 7], we examined the effect of subzero working on the grain refining of austenitic stainless steels. In particular, ultrafine grained AISI 304 stainless steel with an average grain size below 1 μm was obtained by applying the reverse transformation of martensite to austenite, on subzero-worked steel, annealed at low temperatures. Furthermore, a great increase both in the mechanical [6] and in the localized corrosion [8] resistance was found. In order to further increase the strength, it is possible to combine the effects of nitrogen addition and grain refining. In previous works we analyzed the effect of grain size on the mechanical properties [9] and on the wear resistance [10] of a high nitrogen austenitic stainless steel. This paper deals with the corrosion behavior, in particular general corrosion (GC), intergranular corrosion (IGC) and pitting corrosion (PC) of ultrafine-grained high nitrogen austenitic stainless steel. Results are then compared to those of similar measurements on standard AISI 304 steel. The chemical composition of the steel (hereinafter HN) and of the AISI 304 steel under consideration is shown in Table I. After cold working down to 80% thickness reduction, the material was subjected to four different heat

Grain size dependence of mechanical, corrosion and tribological properties of high nitrogen stainless steels

Austenitic stainless steels have been indispensable for the progress of technology during the last 80 years. Due to the cost of nickel and to the prospective of allergic reactions caused by this element, more and more laboratories and industries are trying to develop a new class of austenitic stainless steels with a low nickel content. In order to maintain the austenitic microstructure, nickel reduction is balanced with nitrogen addition. Nitrogen addition to austenitic stainless steels is also very effective for improving yield strength and corrosion resistance without reducing ductility and toughness. In order to further increase the strength, it is possible to combine the effect of nitrogen addition and grain refining. The purpose of this study is to examine the relationship between microstructures and mechanical, corrosion and tribological properties of a high nitrogen stainless steel with an ultrafine grained structure.

The influence of atmospheric humidity and grain size on the friction and wear of AISI 304 austenitic stainless steel

The tribological properties of a ultra-fine AISI 304 austenitic stainless steels obtained by means of a martensitic transformation and subsequent austenite reversion are reported. The effects of the grain size on the wear resistance of such material is, for the first time, investigated as a function of the atmospheric humidity. Decrease of relative humidity in wear tests of AISI 304 steel produces an increase in weight loss and in the friction coefficient. A beneficial effect of grain refining is also shown with respect to large grain steel in that the finer grain steel produces less initial weight loss and the weight loss with an increase in the humidity is also less pronounced.