Decomposition of supercooled austenite of powder phosphorus steels

The structure of powder phosphorus steels is investigated. New models of isothermal decomposition of super-cooled austenite are suggested which make it possible to predict the kinetics of the γ→α transformation in the pearlite and bainite temperature ranges. It is shown that mechanical alloying has a favorable effect on the formation of structure in steels. Grain disintegration accelerates the decomposition of austenite mainly due to the growth of the specific surface. At the same time, the contribution of 0.65–1.1% phosphorus additives to the process is low. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 3–7, May, 1999.     

High-strength crack-resistant concentration-inhomogeneous powder nickel steels

Conclusion The transformation of metastable austenite into strain martensite in concentration-inhomogeneous powder nickel steels under a load promotes an increase in their strength and crack resistance. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 28–32, November, 1999.     

Effect of combined treatment on the stability of austenite and the mechanical properties of chromium-manganese steels

The phase composition, the stablity of austenite with respect to the γ→α′ transformation under a load, and the mechanical properties in torsion of Cr−Mn steels 17Kh13G7S, 20Kh14G7, and 40Kh14G7 with metastable austenite after straining and heat treatment by various regimes are investigated. Methods for a combined treatment of Cr−Mn steels are suggested in order to provide a high level of their strength and ductility properties. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 18–21, January, 1997.     

Specific features of low-temperature phase transformations in nitrogen-Containing Cr-Mn-based steels

Physical and mechanical properties of nitrogen-containing austenitic steels of different alloying systems have been studied at temperatures from −196 to 700°C in the quenched state. It has been found that nitrogen-containing Cr-Mn-based steels undergo a paramagnetic to antiferromagnetic ordering (with a manifestation of invar properties), the ΔE effect, and a resistivity anomaly. It has been shown that the behavior of the temperature dependence of the yield stress in nitrogen austenite is determined by several factors. Along with specific features of the dislocation structure determined by a low energy of stacking faults, the strengthening of nitrogen austenite is influenced by its magnetic state.     

High strength austenitic steels

Conclusions Transitional steels have an unusual combination of mechanical properties. They may have a low yield stress (20–40 kg/sq. mm=28, 400–56, 900 psi) and a high tensile strength (100–200 kg/sq mm=142, 000–284, 000 psi). The mechanical properties of these steels depend mainly on the extent to which austenite decomposes into martensite in deformation, and on the fracture strength [resistance to failure under the action of normal stresses], of the martensite formed. When martensite produced by quenching is present in the initial steel, its rupture resistance also determines the mechanical properties of the steel.In tensile tests on transitional nickel steels containing 0. 26% carbon, initially purely austenitic, there was a considerable plastic deformation (elongation up to 60%); fracture occurs without necking, and the elongation exceeds the reduction of area. In spite of considerable preliminary plastic deformation, the failure is of the brittle type.3. It has been confirmed that in steels containing unstable austenite, a combination of strength and ductility can be achieved, unattainable in steels of other structural categories.     

Microstructure and mechanical properties of austenitic stainless steels after cold rolling

The effect of austenite stability on the evolution of microstructure and mechanical properties of three austenitic stainless steels during cold rolling has been studied. Samples of different grain sizes have been used to characterize the microstructures during deformation. In the case of 304/8% Ni and 304/10% Ni stainless steels, the transformation microstructures consist of mechanical twins: ε-martensite and α′-martensite. No hexagonal close-packed (hcp) ε-martensite was detected in 316 stainless steel. The volume fraction of α′-martensite formed increases with increasing strain in 304 and 316 stainless steels for a given grain size. The amount of α′ phase increases with a decrease in grain size in 304 stainless steel, while the formation of this phase has been found to be grain size insensitive in 316 stainless steel. The strain-hardening behavior exhibited by the three stainless steels used in this study indicates the contribution of both α′-martensite and grain size strengthening in the case of both 304 stainless steels, while only grain size contribution was found in the case of 316 stainless steel.     

Special Features of Steels Alloyed with Nitrogen

The role of nitrogen as an alloying element that stabilizes austenite in steels and makes it possible to replace nickel, manganese, and other austenization promoters without deterioration of mechanical and special properties of the metal is considered. Put into practice this could reduce the volume of mining of the mentioned elements. Methods for introducing nitrogen into iron alloys are described. The mechanical properties of nitrogen-bearing steels are considered. It is shown that such steels with a structure of nitrogen martensite possess a specific strength under static and cyclic loads no worse than the strength of light structural alloys and have better fracture toughness and technological properties than the latter. The replacement of light alloys by such steels should reduce the consumption of energy in the production.     

Effect of prior hot rolling on the microstructures and mechanical properties of duplex stainless steel containing tempered martensite and ferrite

A duplex structure of δ-ferrite and lath martensite with interlath retained austenite film is developed in this study by modifying the alloy addition. The presence of δ-ferrite can further strengthen the grain refinement of austenite during hot rolling. As a result, the amount of retained austenite is enhanced. Tempered martensite embrittlement occurs due to the decomposition of retained austenite, and grain refinement can in fact ameliorate the tempered martensite embrittlement by delaying the onset of the embrittlement to a higher temperature. The combined effect of uniform and small grains as well as a large amount of retained austenite provides a further increase in the mechanical properties. After identical tempering treatments, all mechanical properties measured in the as-rolled condition were found to be higher than those of direct quenching without rolling. After hot rolling, the increase in the hardness and tensile strength was not accompanied by a drop in the ductility and toughness.     

Austenitic-ferritic stainless steels: A state-of-the-art review

Austenitic-ferritic stainless steels, more commonly known as duplex stainless steels, or DSS for short, consist of two basic phases. One is austenite, A, and the other is ferrite, F, present in about equal amounts (but not less than 30% each). The two phases owe their corrosion resistance to the high chromium content. Compared to austenitic stainless steels, ASS, they are stronger (without sacrificing ductility), resist corrosion better, and cost less due to their relatively low nickel content. DSS can be used in an environment where standard ASS are not durable enough, such as chloride solutions (ships, petrochemical plant, etc.). Due to their low nickel content and the presence of nickel, DSS have good weldability. However, they have a limited service temperature range (from −40 to 300°) because heating may cause them to give up objectionable excess phases and lower the threshold of cold brittleness in the heat-affected zone of welded joints. State-of-the art DSS are alloyed with nitrogen to stabilize their austenite, and in this respect the nitrogen does the job of nickel. Also, nitrogen enhances the strength and resistance to pitting and improves the structure of welds. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 20–29, October, 1997.     

Influence of nickel and molybdenum on the phase stability and mechanical properties of maraging steels

The effect of nickel and molybdenum concentrations on the phase transformation and mechanical properties of conventional 18Ni(350) maraging steel has been investigated. Both of these elements act as strong austenite stabilizers. When the concentration of molybdenum or nickel is greater than 7.5 or 24 wt %, respectively, the austenite phase remains stable up to room temperature. In both molybdenum- and nickel-alloyed steels, the austenite phase could be transformed to martensite by either dipping the material in liquid nitrogen or subjecting it to cold working. When 7.5 wt% Mo and 24 wt% Ni were added in combination, however, the austenite phase obtained at room temperature did not transform to martensite when liquid-nitrogen quenched or even when cold rolled to greater than 95% reduction. The aging response of these materials has also been investigated using optical, scanning electron, and scanning transmission electron microscopy.     

Effects of cooling time and alloying elements on the microstructure of the gleeble-simulated heat-affected zone of 22% Cr duplex stainless steels

The effects of austenite stabilizers, such as nitrogen, nickel, and manganese, and cooling time on the microstructure of the Gleeble simulated heat-affected zone (HAZ) of 22% Cr duplex stainless steels were investigated. The submerged are welding was performed for comparison purposes. Optical microscopy (OM) and transmission electron microscopy (TEM) were used for microscopic studies. The amount of Cr₂N precipitates in the simulated HAZ was determined using the potentiostatic electrolysis method. The experimental results indicate that an increase in the nitrogen and nickel contents raised the δ to transformation temperature and also markedly increased the amount of austenite in the HAZ. The lengthened cooling time promotes the reformation of austenite. An increase in the austenite content reduces the supersaturation of nitrogen in ferrite matrix as well as the precipitation tendency of Cr₂N. The optimum cooling time from 800 to 500 °C (Δt _8/5) obtained from the Gleeble simulation is between 30 and 60 s, which ensures the austenite content in HAZ not falling below 25% and superior pitting and stress corrosion cracking resistance for the steels. The effect of manganese on the formation of austenite can be negligible.     

Decomposition of supercooled austenite in steels of the 40Kh type

Conclusions In steels alloyed with strong carbide-forming elements (vanadium, niobium, and titanium) the rate of decomposition of supercooled austenite depends on the type and ratio of atoms of the carbide-forming elements bound in undissolved carbides and dissolved in the solid solution. This should be taken into account in selecting the optimal cooling rate for steels after austenitizing in order to obtain a given combination of mechanical properties.S. M. Kirov Ural Polytechnic Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 56–57, August, 1981.     

Mechanical properties of injection molded Fe-6% Ni-0.4% C steels with varying Mo contents of 0.5 to 2%

In our previous studies, sintered and heat-treated alloy steels (Fe−6Ni−0.5Mo−0.4C (mass%)) produced by a MIM process showed excellent mechanical properties of 2000 MPa tensile strength and 5% elongation. This was attributed to the solid solution strengthening and the mezzo-heterogenous microstruture, which consisted of martensite or retained austenite (Ni and Mo rich phases) surrounded by a network of tempered martensite. This study has been performed to clarify the effect of Mo on the mezzo-heterogeneous microstructure and the mechanical properties of MIM processed and sintered alloy steels (Fe−6Ni−0.4C) with varying Mo content (0.5–2 mass%). The tensile properties of the heat-treated steels with added 2 mass% Mo were lower than those of the steels with added 0.5 mass% Mo. The reduction in the tensile properties, particularly the appearance of large pores formed at the original location of Mo power through the transient liquid phase formation and the low hardness of the matrix, was due to the low sintered density. By using mechanically milled fine Ni and Mo powders, the heat-treated steel (Fe−6Ni−2Mo−0.4C) showed excellent properties, including tensile strength of 1800 MPa and ductility of 2.2% elongation. This article is based on a presentation made in the symposium “The 3rd KIM-JIM Joint Symposium on Advanced Powder Materials”, held at Korea University, Seoul, Korea, October 26–27, 2001 under auspices of The Korean Institute of Metals and Materials and The Japan Institute of Metals.     

Effect of bainitic decomposition products of austenite on the mechanical properties of quenched and tempered structural steels

Conclusions Precipitation of nonmartensitic decomposition products of supercooled austenite leads to uneven mechanical properties in different sections of low alloy structural steels directly after quenching and after high-temperature tempering.At the same ultimate strength, the yield strength and ductility decrease and the yield point on the stress-strain diagram disappears.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 51–53, June, 1976.     

Effects of phosphorous addition on mechanical properties and retained austenite stability of 0.15C−1.5Mn−1.5Al TRIP-aided cold rolled steels

The effects of a P addition on the mechanical properties and austenite stability are investigated for 0.15C−1.5Mn−1.5Al TRIP-aided cold-rolled steels containing 0.05 and 0.1 wt.% of P. The strength and retained austenite fraction are increased by an increment of the P content. The strengthening of P-added TRIP-aided steel partially comes from the solid-solution hardening effect of P, and a higher fraction of strain-induced martensite plays an important role as well. The elongation of steel containing 0.1 wt.% P is diminished compared with that containing 0.05 wt.% P. This is attributed to the lower mechanical stability of retained austenite in TRIP-aided steel containing 0.1 wt.% of P, which inhibits persistent work hardening during deformation.     

Special features of the TRIP effect in powder steels with nonuniform concentration and low nickel content

The term “TRIP effect” has been suggested by V. Zakei and I. Parker for denoting deformation austenite— martensite transformation. At the present time, the term is applied to steels of the austenite—martensite class that have phase transformations under a load. TRIP steels exhibit the best combination of strength and toughness. The present work shows the possibility of realizing the TRIP effect in powder steels at a relatively low concentration of the alloying element (nickel) due to its nonuniform distribution. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 15–19, August, 1997.     

Microstructural Characteristics and Mechanical Properties of Low-Alloy,Medium-Carbon Steels After Multiple Tempering

The microstructure and mechanical properties of NiCrMoV-and NiCrSi-alloyed medium-carbon steels were investigated after multiple tempering. After austenitising, the steels were hardened by oil quenching and subsequently double or triple tempered at temperatures from 250 to 500 °C. The samples were characterised using scanning electron microscopy and X-ray diffraction, while the mechanical properties were evaluated by Vickers hardness testing, V-notched Charpy impact testing and tensile testing. The results showed that the retained austenite was stable up to 400 °C and the applied multiple tempering below this temperature did not lead to a complete decomposition of retained austenite in both steels. It was also found that the microstructure, hardness and impact toughness varied mainly as a function of tempering temperature,regardless of the number of tempering stages. Moreover, the impact toughness of NiCrMoV steel was rather similar after single/triple tempering at different temperatures, while NiCrSi steel exhibited tempered martensite embrittlement after single/double tempering at 400 °C. The observed difference was mainly attributed to the effect of precipitation behaviour due to the effect of alloying additions in the studied steels.     

Effect of thermal-strain treatment of steel on its structural state and level of properties

At present the quality of steel is often improved by thermomechanical treatment (TMT). It is of interest to establish the effect of the state of austenite in strain caused by TMT on the properties of steels. This paper is devoted to the effect of various thermal-strain treatment regimes on the properties and structure of steels 25GSR and 20 with different stabilities of austenite. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 16–18, January, 1997.     

Effect of the manufacturing technology on the structure and properties of heat-treatable powder steels

High-energy disintegration of powder charges (the method of mechanical alloying) in which the initial components interact at a higher substructural level than in conventional mixing provides qualitatively new powder materials with a high degree of homogeneity and dispersity of the structure. The possibilities of the given method for a radical improvement of the quality of low-alloy heat-treatable steels virtually have not been studied because of the absence of data on the special features of polymorphic transformations in mechanically alloyed powder materials based on iron. This hampers realization of the advantages of powder metallurgy over other less efficient technologies in the field of manufacturing high-strength structural parts. In order to substantiate the choice of heat-treatment regimes of mechanically alloyed materials, the authors of the present paper studied the special features of the isothermal γ→α transformation and the structure and properties of low-alloy steels after sintering and heat treatment. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 20–22, August, 1997.     

Phase composition and hardening of steels of the Fe-Cr-Ni-Co-Mo system with martensite-austenite structure

The phase composition and mechanical properties of maraging steels of the Fe-Cr-Ni-Co-Mo system are studied as a function of the alloying and of the temperatures of quenching and aging. The intermetallic phases strengthening martensite in different aging stages are determined. The degree of the hardening and the variation of the impact toughness at cryogenic temperatures are compared for steels with different structures (martensite and martensite-austenite) in the stages of maximum hardening and overaging. The effect of retained and reverted austenite on the resistance to crack propagation under impact loading is determined for steels with martensite of a different nature and amount of hardening phases. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 32–37, April, 2007.