Influence of nitrogen-alloying on the tempering properties of the martensitic stainless steel 00Cr13Ni4Mo

The mechanical and corrosive properties of 00Cr13Ni4Mo （S13 -4N） were tested and compared with those of 00Cr13Ni6Mo （S13 -6）. The effects of nitrogen on the properties of the steels were analyzed. The results of the tensile and corrosion tests show the strength,the ductility,and the pitting corrosion resistance of S13 -4N are higher, lower and poorer than those of S13 -6 respectively, when tempered at a temperature below 550 ℃and vice versa when the tempering temperature is higher than 550℃. The results of the X-ray diffraction （XRD） and the electron backscattered diffraction （EBSD） analyses reveal that inversed austenite appears at 550℃ and the amount of it peaks at 600 ℃ with the best ductility. And the total amount of the inversed austenite in S13 -6 is more than that in S13 -4N in different forms. Nitrogen performs better in terms of stabilizing inversed austenite while nickel is more favorable for forming inversed austenite, the amount and stability of which affect the ductility remarkably. The reason for the embrittlement of S13 -4N at 450℃ can be the result of carbide and nitride precipitating at grain boundaries.

Effects of Tempering Temperature and Mo/Ni on Microstructures and Properties of Lath Martensitic Wear-Resistant Steels

The tempering behavior was experimentally studied in lath martensitic wear-resistant steels with various Mo/Ni contents after tempering at different temperatures from 200to 600℃.It is shown that a good combination of hardness（HV）（420-450）and-20℃impact toughness（38-70J）can be obtained after quenching and tempering at 200-250 ℃.The microstructure at this temperature is lath structure with rod-like and/or flake-likeε-carbide with about 10nm in width and 100nm in length in the matrix,and the fracture mechanism is quasi-cleavage fracture combining with ductile fracture.Tempering at temperature from 300to 400℃results in the primary quasi-cleavagefracture due to the carbide transformation from resolved retained austenite and impurity segregation between laths or blocks.However,when the tempering temperature is higher than 500℃,the hardness（HV）is lower than 330 and the fracture mechanism changes to ductile fracture due to the spheroidization and coarsening of cementite.Additions of Mo and Ni have no significant effects on the carbides morphologies at low tempering temperatures,but improve the resistance to softening and embrittling for steels when tempered at above 350℃.

Mechanical properties of a microalloyed bainitic steel after hot forging and tempering

Mechanical properties of a newly developed microalloyed bainitic steel were investigated after the hot forging, air cooling and tempering process. The microstructure of the as forged bainitic steel mainly consists of granular bainite and -20 vol. % martensite. The fraction of retained austenite remains unchanged until tempering at 200 ℃, above which it decreases significantly. The increase of tempering temperature leads to decreases of both ultimate tensile strength and total elongation but decreases of both yield strength and reduction of area. The maximum and mini- mum values of impact toughness were observed after tempering at around 200 and 400 ℃, respectively. These effects are mainly attributed to the decomposition of martensite/austenite con stituents and the tempering effects in martensite. The tempering of the forged bainitic steel at around 200 ℃ results in an excellent combination of strength and toughness, which is comparable to that of the conventional quenched and-tempered 40Cr steel. Therefore, low-tempering treatment coupled with post-forging residual stress relieving is a feasible method to further improve the mechanical prooerties of the bainitic foging steel.

Thermal stability of retained austenite and mechanical properties of medium-Mn steel during tempering treatment

The thermal stability of retained austenite（RA）and the mechanical properties of the quenched and intercritical annealed 0.1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained austenite during tempering within the range from 200 to 500°C were studied by X-ray diffraction（XRD）,transmission electron microscopy（TEM）and tensile testing.The results showed that there was a slight decrease in the RA volume fraction with increasing tempering temperature up to 400°C.This caused a slight increase in the ultimate tensile strength（UTS）and a slight decrease in the total elongation（TE）;thus,the product of UTS to TE（UTS×TE）as high as 31GPa·% was obtained and remained nearly unchanged.However,aportion of the RA began to decompose when tempered at 500°C and thus caused a^35% decrease of the RA fraction and a^16%decrease of the value of UTS×TE.It is concluded that the ultrafine lamellar duplex structure is rather stable and the excellent combination of strength and ductility could be retained with tempering temperature up to 400°C.Thus,thermal processes such as galvanization are feasible for the tested steel provided that their temperatures are not higher than 400°C.

Effect of rhenium on the microstructure and mechanical behavior of Fe–2.25Cr–1.6W–0.25V-0.1C bainitic steels

A new ferritic creep resistant steel has been developed by eliminating Nb and adding 1.5 mass % Re to a ferritic steel grade T/P23 with the aim of enhancing its mechanical properties at high temperature.Cast ingots of both steels, new grade and ASTM T/P 23, were hot rolled at 900℃ and then submitted to a thermal treatment consisting of solubilization at 1050℃ and tempering at 700℃. Tempered bainitic microstructures obtained contain second phases reinforcing carbide particles, mainly M_6C and M_（23）C_6 at the boundaries of both, prior austenite grains and bainitic ferrite laths, as well as MC within the grains. Mechanical properties at temperatures ranging from 540 to 600℃ were studied by strain-ratechange tests in compression at strain rates between 10^（-7） and 10^（-4）s^（-1）. These tests showed high stress exponents（n ≥ 20） and activation energies（Q ≈ 400 k J/mol） for both alloys, which were associated with a dislocation movement mechanism with a strong interaction between dislocations and precipitates. On the other hand, a creep exponent of 5 was derived for the stress dependence of minimum creep rate from conventional-type creep tests at 600℃. Although this stress exponent is usually related to a dislocation climb controlled creep mechanism, remarkable microstructural degradation observed with increasing creep time makes difficult to elucidate the true deformation mechanism controlling creep.