Showing papers on "Tempering published in 1994"

Atom probe field ion microscopy study of the partitioning of substitutional elements during tempering of a low-alloy steel martensite

Abstract: The partitioning behavior of Mn and Si at the cementite/ferrite interface during tempering of Fe-C-Si-Mn steel martensite has been studied by atom probe field-ion microscopy (APFIM). It has been shown that cementite can form without partitioning of Si and Mn during the early tempering stage at a low temperature. The atom probe compositional analysis shows no evidence of segregation or of concentration spikes of substitutional elements at the interface. This suggests that the early stage of cementite growth occurs by paraequilibrium mode and is controlled only by C diffusion in the matrix. In addition, significant C concentration fluctuations are measured in the as-quenched condition. The onset of partitioning of both Si and Mn occurs after prolonged time or by increasing the tempering temperature.

Synthesis and properties of α″‐Fe16N2 in magnetic particles

Abstract: The α‘‐Fe16N2 phase was synthesized by a nitriding, quenching, and tempering process starting from Fe2O3 Acicular γ‐Fe2O3 was first reduced to α‐Fe under H2, then it was converted to γ‐austenite at 650–700 °C by nitriding using a NH3‐H2 mixed gas A martensitic transformation to α’‐martensite occurred when the sample was quenched in liquid nitrogen Finally, the α‘‐Fe16N2 phase was formed from the α’‐martensite by tempering in the temperature range 120–200 °C In order to increase the extent of the martensitic transformation, acicular Fe2O3 particles with 10–15 at % MnO2 were prepared by oxidizing Fe++ and Mn++ in alkaline aqueous solution When a quenched sample, which contains martensite, α‐Fe, and austenite, was tempered at 200 °C for different times, the magnetic moment first increased (transformation of α’‐martensite to α‘‐Fe16N2); the highest magnetic moment was obtained at about 60 min of tempering Using even longer tempering times, the magnetic moment decreased (decomposition of α‘‐Fe16N2 to α‐F

Isothermal formation of quasicrystalline precipitates and their effect on strength in a 12Cr-9Ni-4Mo maraging stainless steel

Abstract: A new type of maraging steel, Sandvik 1RK91, intended for use in surgical applications has recently been developed at Sandvik Steel, Sandviken, Sweden. In the present article, a comparison was made between this steel and a traditional maraging steel with respect to mechanical and physical properties after tempering treatments in the temperature range 375 °C to 580 °C. The tempering behavior was studied by analyzing hardness, resistivity, and volume fraction of magnetic phase. Sandvik 1RK91 showed a remarkably high microstructural stability against overaging, softening being observed only at 580 °C. In comparison with the reference steel, the hardness of Sandvik 1RK91 was found to be considerably higher in the whole range of temperatures, with the maximum values corresponding to a strength of 3000 MPa for the smallest dimensions of wire. A detailed microstructural investigation of precipitation reactions in Sandvik 1RK91 revealed a new type of precipitate at 475 °C that was found to be of a quasicrystalline nature. These precipitates, which were found to give the major contribution to particle strengthening after tempering at 475 °C, showed a close resemblance to intermetallic trigonalR phase with a chemical composition of about 50 pct molybdenum, 15 pct chromium, 30 pct iron, and 5 pct silicon. The similarity between the quasicrystalline precipitates andR phase was reflected as a second-order transition occurring at about 525 °C. The extremely high strength in 1RK91 can be ascribed to the quasicrystalline nature of the precipitates because of impeded particle shearing.

Corrosion of Carbon Steels, Stainless Steels, and Titanium in Aqueous Lithium Bromide Solution

Abstract: Effects of lithium bromide (LiBr) concentration, pH, temperature, exposure time, and the action of some inhibitors on corrosion of several carbon (C) steels, stainless steels (SS), and a titanium (Ti) alloy were studied. Corrosion rates were determined by the polarization resistance method and compared to rates determined by weight-loss measurements. Pitting potentials (Ep) were evaluated in neutral LiBr solution and with different inhibitors. Pit density and average pit depth depended on the metal tested, with lowest values for Ti, the next lowest values for type 316 SS (UNS S31600), and the highest values for UNS G41350 tempered steel.

Influence of tempering temperature on stability of carbide phases in 2.6cr-0.7mo-0.3v steel with various carbon content

Abstract: The present work evaluates the influence of the bulk carbon content (0.1, 0.006, and 0.005 wt pct) and tempering temperature (823, 853, and 913 K) on stability, chemical composition, and size of carbide particles in 540 ks tempered states of 2.6Cr-0.7Mo-0.3V steel. The scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDXS) and electron diffraction methods were used to analyze the carbide particles. A characteristic energy-dispersive X-ray (EDX) spectrum can be attributed to each of the identified carbides. The MC carbide is stable in all experimental states. The phase stability of Fe-Cr-rich carbides increased in the order e, Fe3C → M3C → M7C3, with tempering temperature increasing. In steels with higher carbon content tempered at low temperature, M23C6 carbide was also noted. The Mo2C and M6C carbides were not observed. It was shown that the decrease of the bulk carbon content has the same influence on the carbide phases stability as the increase of the bulk vanadium content at the unchanged Cr, Mo, C bulk contents and tempering temperature. Similarly, the decrease of tempering temperature has the same influence on the carbide phases stability as the decrease of the bulk Cr content at the unchanged V, Mo, and C bulk contents.

Effect of Austenitizing Temperature on Microstructure and Mechanical Properties of 12%Cr Steel

Abstract: The effect of austenitizing temperature were investigated on the microstructure and mechanical properties of 12%Cr steel. Low-temperture austenitizing below 1000°C induced the carbide coarsening during subsequent tempering at 750°C for 1 hr due to the nucleation effect of undissolved M23C6. The large and spheroidized carbides enhanced the subgrain growth. On the other hand, the complete dissolution of M23C6 above 1000°C caused the fine carbide formation on lath boundaries, which retarded the subgrain growth during tempering. Furthermore, the dissolution of Nb(C, N) above 1100°C enhanced the tempering resistance through increasing the stability of lath morphology and reducing the growth rate of M23C6. The increase in strength with increasing austenitizing temperature was attributed to the fine carbide distribution and the high dislocation density. Further, as the austenitizing temperature increased, the impact energy markedly reduced, due to the large proir austenite grain size and the high strength.

Method for producing ultra high strength, secondary hardening steels with superior toughness and weldability

Abstract: High strength steel is produced by a first rolling of a steel composition, reheated above 1100° C., above the austenite recrystallization, a second rolling below the austenite recrystallization temperature, water cooling from above Ar3 to less than 400° C. and followed by tempering below the Ac1 transformation point.

High density ferrous power metal alloy

Abstract: A method for producing high density and/or high surface density ferrous powder metal parts has the steps of: compacting a iron-containing powder substantially free of graphite at room temperature and at about 40-50 tsi; sintering the green compact in an inert, non-oxidizing environment at a temperature of about 2050°-2300° F.; repressing the sintered compact at room temperature at about 60 tsi; carburizing the repressed compact at high temperature to form a layer of relatively high carbon concentration to a depth of at least about 0.010 inches; and immediately quenching the hot carburized compact followed by a tempering treatment.

Influence of microstructure on hydrogen embrittlement behaviour of 2·25Cr–1 Mo steel

Abstract: The hydrogen embrittlement of Cr–Mo steel has been studied. The effects of tempering temperature on the hydrogen diffusivity and hydrogen assisted cracking, without external stress, are investigated. Hydrogen permeation and trapping, for the various microstructural conditions, were measured using electrochemical equipment. The microprecipitate distribution in the steel was observed using transmission electron microscopy. The steel, which was heat treated to give a variety of microstructures, was cathodically hydrogen charged and the critical microstructural sites for hydrogen induced cracking examined. Cracks initiated by the charging treatments were found to nucleate at MnS interfaces. Hydrogen diffusivity and trapping are strongly dependent on the tempering parameters.MST/1947

Cementite precipitation during tempering of martensite under the influence of an externally applied stress

Abstract: The precipitation of cementite under the influence of an externally applied stress, during the tempering of martensite in steels, is investigated using transmission electron microscopy. The stress appears to favour the development of particular crystallographic variants of cementite in any given plate of martensite. Hence, a Widmanstatten array of cementite particles in a normally tempered sample changes to an array consisting of just one variant in stress-tempered samples. The results are discussed in the context of the mechanism of carbide precipitation during the lower bainite reaction.

The effects of double austenitization on the mechanical properties of a 0.34C containing low-alloy Ni-Cr-Mo-V steel

Abstract: This article considers five different microstructures of a tempered martensitic 0.34C, 3Ni-1.3Cr-0.4Mo-0.1V steel through various heat treatments, including double austenitization (DA) treatments, and how the impact toughnesses are influenced by microstructure. Of the four mechanisms considered to explain the beneficial effect of DA treatment, the roles of retained austenite, grain-boundary embrittlement by impurity segregation, and matrix flow stress are discounted. The 50 pct fracture appearance transition temperature (FATT) of this steel is found to be dependent on both the grain size and the carbide dissolution. The conventionally treated steel contains mainly platelike M3C carbides. The DA treatment helps to dissolve VC carbides and coarsen and spheroidize M3C carbides in favor of the precipitation of short rodlike M7C3 carbides with a lower aspect ratio. The improvement of impact toughness (upper shelf energy, ductile-to-brittle transition temperature (DBTT), and lower shelf energy) by DA treatment, explained in detail, is attributed to a change of this material’s tensile and work-hardening behavior affected by a variation of carbide morphology.

Production of high strength steel pipe having excellent sulfide stress corrosion crack resistance

Abstract: PURPOSE:To obtain a steel pipe having excellent SSCC resistance and strength by acceleration-cooling after hot pierce-rolling a specific composition of steel billet, successively, mandrel-rolling to make a seamless steel pipe and, thereafter, immediately quenching and tempering. CONSTITUTION:The composition of the steel is made to be, by wt.%, 0.15-0.4 C, 0.1-1 Si, 0.3-1 Mn, 0.1-1.5 Cr, 0.3-1 Mo, 0.0005-0.003 B, 0.01-0.1 Al, 0.003-0.01 N, <=0.015 P, <=0.005 S, and further, one or more kinds of 0.01-0.05 V, 0.01-0.05 Nb and 0.01-0.03% Ti and the balance Fe. This steel billet is heated to 1250-1350 deg.C and the Mannesmann type seamless rolling is applied to execute hot-piercing. This pipe is acceleration-cooled to the recrystallizing temp. +50 deg.C to the recrystallizing temp. +100 deg.C. Rolling reduction is executed to this pipe by the mandrel-rolling at <=30% and at the recrystallizing temp. to the recrystallizing temp. +30 deg.C, and finished at <=Ar3 transformation point to make the seamless steel pipe. This pipe is immediately water-hardened and, thereafter, the tempering is executed.

Production of high hardness wear resistant steel excellent in low temperature toughness

Abstract: (57) [Summary] [Object] To provide a method for producing a high-hardness wear-resistant steel having a Brinell hardness of HB500 or more, which has temper softening resistance and low temperature toughness. [Structure] C: 0.30 to 0.50%, Si: 0.40 ˜1.50%, Mn: 0.40 to 1.50%, Cr: 0.10 to 1.50%, Mo: 0.05 to 1.00%, Ti: 0.005-0.050%, Nb: 0.005- 0.050%, B: 0.0005 to 0.0030%, s ol. Al: 0.01 to 0.10%, N: 0.0010 Steel containing 0.1 to 0.0060% as a basic component, optionally containing one or more of Cu, Ni, V, and Ca, with the balance being Fe and unavoidable impurities. After heating to 00 to 1250 ° C. and hot rolling, 10 to 10 ° C./sec or more cooling rate from a temperature of A 3 transformation point or more. A quenching process of cooling to a temperature of 0 ° C. or lower is performed, and then a tempering process is performed at a temperature of 150 ° C. or higher and 450 ° C. or lower.

Effects of heat treatment on microstructure and mechanical properties of Cr–Mo–3·5Ni–V steel

Abstract: Heat treatments have been carried out on an annealed low alloy Cr–Mo–3·5Ni–V steel (J-steel) to investigate the effects of various normalising and hardening temperatures, before tempering, on the mechanical properties. It was observed that the 0·2% proof stress increased, whereas the impact toughness, as measured by the increase in transition temperature, decreased with increasing hardening temperature for both normalised and non-normalised material. For a constant hardening temperature, the 0·2% proof stress passed through a minimum and the Charpy upper shelf energy passed through a maximum, at a normalising temperature of 890°C. The variation in properties has been correlated with the observed variation in martensite packet size, dislocation density, precipitate size, and volume fraction. It was found that a 0·2% proof stress above 1100 MN m−2, and a Charpy impact energy of greater than 50 J at −40°C located within the upper shelf energy region, was attainable with the following heat treatment:...

High strength steel excellent in sulfide stress cracking resistance and its production

Abstract: PURPOSE: To produce high strength steel excellent in sulfide stress cracking resistance by subjecting steel having a specified compsn. contg. Mo, Al, Nb, Ti or the like to specified heat treatment to form its structure into a martensite and thereafter tempering it at the AC 1 transformation point or below. CONSTITUTION: Steel contg., by mass, 0.10 to 0.35% C, 0.01 to 0.50% Si, 0.10 to 0.6% Mn, ≤0.005% S, ≤0.015% P, 0.30 to 1.0% Mo, 0.005 to 0.1% Al, 0.01 to 0.1% Nb and 0.005 to 0.04% Ti, furthermore contg., Ti≥3.4N, ≤0.006% N and 0.0008 to 0.0016% B and moreover contg., at need, prescribed amounts of Cr, V, Co, Zr, rare earth elements, Ca or the like, and the balance substantial Fe is subjected to short time reheating to 930 to 1000°C for ≤5min at ≥3°C/sec heating rate and is hardened to form its structure into a martensite. This steel is tempered at the AC 1 transformation point or below. In this way, the steel having a martensitic structure of grains whose size is finer than No.9.5 (ASTMNo.) and having 84 to 100kgf/mm 2 yield strength can be obtd. COPYRIGHT: (C)1994,JPO

Physically vapor deposited coatings on cermets: performance and wear phenomena in interrupted cutting

Abstract: Cermets are titanium based cutting materials with high resistance against abrasive and chemical reactions take place between the binder materials and steel. Recent developments of substrates and the use of physically vapor deposited coatings focus on further improvement of the wear and fracture resistance of cermets in order to enlarge the field of application towards tempered and austenitic steels and towards more rigorous interrupted cutting operations. Binary and ternary titanium based hard coatings deposited onto different cermet substrates by physical vapor deposition (PVD) processes were examined in interrupted cutting of tempered, case hardening and austenitic steels. PVD (Ti,Zr)N and Ti(C,N) coatings improve tool life in the machining of tempered steel. The shape of the tool life curves is linked to the strength of the work material as a function of temperature. The PVD TiN coating reduces flank wear of the tools but does not increase the resistance against chipping and breakage in machining of case hardening steel. The thermal conductivity of the cermets is of great importance for performance in interrupted cutting. Severe and early decoating of the tools occurred in machining of austenitic steel owing to adhesive and cohesive coating failure.

Quasicrystalline precipitation hardened metal alloy and method of making

Abstract: A precipitation hardened metallic alloy is provided in which the strengthening is based on the precipitation of particles. The strengthening particles have a quasicrystalline structure, said structure being essentially maintained at aging times up to 1000 h and tempering treatments up to 650° C., the strengthening involving an increase in tensile strength of at least 200 MPa.

Development of 590-MPa Class High Tensile Strength Steel with Superior HAZ Toughness by Copper Precipitation Hardening.

Abstract: Through the study of a chemical composition system capable of assuring high strength without impairing HAZ toughness, 590-MPa high tensile strength steel for offshore structure was developed, and its HAZ toughness improvement mechanism was clarified. The high-copper and ultra-low niobium steel provides superior HAZ toughness with medium heat input in -60°C Charpy test and -30°C CTOD test and with high input in -60°C Charpy test. Why HAZ toughness comparable to that of 490-MPa high tensile strength steel is obtained may be considered as follows. The precipitation of copper occurs much later than other precipitation-hardening elements like niobium and vanadium. Copper precipitated by tempering for strengthening goes into solid solution in the heating process of the HAZ. Copper precipitates little on cooling (and heating) during the subsequent welding thermal cycle. The embrittlement of HAZ by the precipitation hardening of copper thus does not take place. This allows the 590-MPa steel to be welded without preheating and cracking, just like 490-MPa steel.

Direct observation of boron segregation at grain boundaries in astrology by 3D atomic tomography

Abstract: Rapid, continuous heat treatment for control of microstructure is a widely used technology for ferrous alloys For example, induction and, to a lesser extent, laser and electron beam methods are common for the surface hardening of steels [1] Induction heating has also been applied for tempering of quench-hardened steels, solution annealing of stainless steels, and recrystallization annealing of cold worked carbon, electrical, and stainless sheet steels [1,2] On the other hand, rapid heat treatment applications have been limited for nonferrous materials Induction heating has been applied for full (and partial) annealing of cold rolled, non-heat treatable aluminum alloys on a production scale [3] For titanium and titanium aluminide alloys, various rapid heating techniques have been investigated for beta annealing, recrystallization annealing of cold worked sheet alloys, and other special processes, albeit only on a limited laboratory scale and only from an empirical standpoint [4-7] The objective of the present work was to analyze the kinetics of beta grain growth during rapid, continuous heating of a conventional alpha-beta titanium alloy The analysis was based on approximate, closed-form theoretical expressions derived by Bourell and Kaysser [8] and Soper and Semiatin [9] as well as a fully numerical, computer-based approach The problem and approach discussed here differs from previous investigations of grain growth during continuous heating and cooling [10-13], most of which have been for austenite grain growth in the heat-affected zone during welding of steels In this regard, the main features of the present work are (1) the very high heating rates involved, (2) the avoidance of the application of complex numerical intergration schemes, and (3) the avoidance of using isothermal grain growth kinetic data to fit continuous heating results

Stainless steel for engine gasket and its production

Abstract: PURPOSE:To produce a high strength stainless steel most suitable for engine gasket for automobile, etc., and excellent in spring characteristic, fatigue characteristic, and stress corrosion cracking resistance. CONSTITUTION:This steel is a stainless steel for engine gasket, which contains, by weight, 0.1-0.5% C, <=2% Si, <=5% Mn, 11-18% Cr, <=0.01% S, <=0.01% O, 0.01-0.2% N, and <=0.0005% H and in which the metallic structure of <=0.01% space factor of nonmetallic inclusions is composed of tempered martensite and Vickers hardness is regulated to 400-550. Further, if necessary, Al, Mg, Ca, Ni, Cu, Mo, Ti, and Nb are incorporated. Moreover, in order to attain this hardness, hardening heat treatment at 900-1050 deg.C and tempering heat treatment at 150-500 deg.C are carried out after cold rolling.

High toughness and high strength untempered steel and processing method thereof

Abstract: The present invention is concerned about high toughness and high strength untempered steel having the mechanical properties equivalent to or better than those of tempered steel and processing method thereof, more particularly, the high toughness and high strength untempered steel having the tensile strength higher than 90kgf/mm² with the impact toughness higher than 5kgf-m/cm² in the KS 3 specimen, and processing method thereof.

High-strength stainless steel for use as material of fuel injection nozzle or needle for internal combustion engine, fuel injection nozzle made of the stainless steel

Abstract: A high-strength stainless steel for use as a material of a fuel injection nozzle or a fuel injection needle of an internal combustion engine. The stainless steel is an as annealed martensitic stainless steel which exhibits a hardness not less than HRC 58 after quenching and tempering heat-treatment. The limit swaging ratio of said as annealed martensitic stainless steel is not less than 75%. The hardness of said as annealed martensitic stainless steel is not higher than HB 157. Preferably, the number of carbides having sizes of 0.2 μm or less occupies not more than 50% of the total carbides, and wherein the limit swaging ratio of said as annealed martensitic stainless steel is not less than 75% or the hardness of said as annealed martensitic stainless steel is not higher than HB 157. Preferably, the stainless steel has a chemical composition containing, by weight: 0.4 to 0.6% of C; not more than 0.5% of Si; not more than 0.5% of Mn; 8.0 to 13.0% of Cr; 0.1 to 2.0% of one or both of W and Mo in terms of W/2+Mo; one or both of 0.05 to 1.0% of one or both of Nb and V in terms of Nb/2+V, and 0.2 to 2.0% of Co; and the balance substantially Fe and incidental impurities.

Analytical electron microscopy of grain boundaries in high-strength steels

Abstract: Phosphorus could be detected at prior austenite grain boundaries (PAGB) in high-strength alloy steels quenched and tempered at 500°C when using a VG's HB 501 dedicated field emission STEM but not with a conventional JEOL 4000FX STEM No phosphorus was detected at PAGB's in the as-quenched materials or away from PAGB's in tempered materials of either type The grain boundary coverage of phosphorus was, assuming a specimen thickness of 80 nm, 07 monolayers for the 35NiCrMoV rotor steel and 04 monolayers for the AISI 4340 steel The grain boundary concentration of phosphorus, assuming a specimen thickness of 80 nm and a segregated layer thickness of 1 nm, for the 35NiCrMoV rotor steel was 6 wt% and for AISI 4340 4 wt% Compared to the bulk concentration of about 001 wt% this means that the enrichment factor of P to the grain boundaries was several hundred times (610 respectively 370) Our measurements showed no correlation between the stress corrosion crack growth rate and the grain boundary phosphorus concentration The yield strength, however, decreased after tempering while the phosphorus concentration at the grain boundaries increased

High strength and high toughness hot-dip plated steel wire and its production

Abstract: PURPOSE:To develop a high strength and high toughness hot-dip plated steel wire excellent in corrosion resistance by subjecting the hot rolled material of high carbon steel to patenting treatment, thereafter subjecting it to cold wire drawing to form into a steel wire, subjecting only the surface to tempering treatment and subsequently applying plating of Zn or the like thereto. CONSTITUTION:A high carbon steel contg., by weight, 0.7 to 1.2% C, 0.5 to 2.0% Si, 0.2 to 1.0% Mn, 0.02 to 0.07% Al and 0.003 to 0.015% N, furthermore contg. specified small amounts of one or more kinds among Cu, Cr, Ni, Co and W and moreover contg. one or more kinds among V, Nb, Ti and B is subjected to hot rolling to form into a wire rod. After that, it is subjected to patenting treatment or is subjected to patenting treatment after reaustenitization to form into a steel wire rod having a main structure of fine pearlite, which is thereafter subjected to cold wire drawing to form into a steel wire having a required diameter. Next, only the surface of the steel wire is subjected to heating and tempering treatment by the induction hardening method, etc., and is thereafter applied with galvanizing or Zn-Al allay plating. HV/TS, i.e., the average Vickers thickness of the steel wire stock to 50mum toward the center of the steel wire from the boundary of the plating layer and the steel wire stock to the tensile strength TS (kgf/mm ) of the plated steel wire is regulated to <2.

Toughness Deterioration and Its Improvement of Weld HAZ with High Heat Inputs in 780 and 980MPa Class HSLA Steels(Report 2). Metallographic Investigation of M-A Constituent.

Abstract: The internal microstructure, chemical composition, and hardness of M-A (Martensite-Austenite constituent) formed in weld CGHAZ (Coarse Grained Heat Affected Zone) of 780-980 MPa class HSLA steels have been investigated in order to reveal metallographic characteristics of the M-A constituent. The martensite was classified into lath and twin type. The massive M-A constituent had higher C contents, and included more twin type martensite and retained austenite than the elongated M-A. The cementite was found to be classified into a coarse rod type and a dendritic or fine needle type. The coarse rod type is considered to be precipitated directly from the austenite. The dendritic or fine needle type is probably precipitated by the self tempering of martensite.The C content of the M-A constituent was increased with Δt8/5 (cooling time from 1073 to 773 K) and reached 1.3-2.2% for Δt8/5=100-1000s.The hardness of the M-A constituent was increased with C content. The hardness of the massive M-A constituent (Hv-950) was generally higher than that of the elongated M-A constituent (Hv-700). The hardness and C content of the M-A constituent can be reduced significantly by a post weld heat treatment at temperatures from 623 to 773 K.

Mechanical properties of metals : experiments with steel, copper, tin, zinc, and soap bubbles

Abstract: Annealing, hardening, and tempering of metals; using bubbles to model the crystalline structure of metals.

Effects of ni addition on sulfide stress cracking resistance of low alloy tempered martensite steels

Abstract: The detrimental effect of Ni addition on sulfide stress cracking (SSC) resistance has been well known since the beginning of the study on SSC. However, effects of Ni itself and untempered martensite which can be typically formed for Ni bearing steel have not been clearly established. In this paper, the effects of Ni addition on SSC resistance are investigated using constant load tests and DCB tests for low alloy tempered martensite steels. SSC threshold stress (σth) decreases with an increase in Ni content at a constant yield strength. This is probably caused by the fact that fissures work as initiation sites of SSC. On the other hand, an addition of Ni raises slightly the value of KISSC. Each KISSC value is supposed to be determined by the hydrogen content. Hydrogen content when crack propagation stops decreases with an increase in Ni content, while hydrogen content increases in Ni content then the steel starts being exposed to a sour environment. Untempered martensite is cormed during tempering at a temperature just below AC1. Steels with untempered martensite show lower values of σth and KISSC than those without untempered martensite at a constant yield strength. So an addition of Ni promotes the formation of fissures and accelerates SSC initiation even without untempered martensite. Furthermore, untempered martensite decreases even more SSC resistance.