Showing papers on "Tempering published in 1979"

Tempered martensite embrittlement in SAE 4340 steel

Abstract: The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K 1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).

A study of the early stages of tempering in an Fe-1.2 Pct alloy

Abstract: Mossbauer effect spectroscopy has been used to study changes in the microstructure of an Fe-1.22. wt pct C alloy due to tempering between 373 and 523 K. The orthorhombic transition carbide, η-Fe2C, was identified by transmission electron microscopy and the similarity of ∈-carbide electron diffraction patterns to η-carbide diffraction patterns is noted. Systematic changes in the Mosbauer parameters of martensite and austenite are presented for the various stages of tempering. The same amount of C remains randomly dissolved in the retained austenite throughout tempering and some C is retained in the martensite throughout the range of transition carbide formation. Two sets of Mossbauer parameters corresponding to magnetic phases other than martensite and cementite have been found. These parameters may come from η-carbide, but alternative interpretations are presented.

Method for making a cone for a rock bit and product

Abstract: A method is provided for manufacturing tungsten carbide insert-type cutter cones for a rock bit for drilling oil wells and the like. A cone blank is formed from medium to high carbon steel by forging and machining. The cone blank has a generally conical external surface, a generally cylindrical internal bearing cavity, and a circumferentially extending ball bearing race in the bearing cavity. The cone blank is heat treated by quenching and tempering to a desired core hardness. Insert holes are drilled in the external surface of the heat treated cone blank for insertion of tungsten carbide inserts. The surface of the ball race is selectively hardened by heating and quenching for forming a surface layer having a higher hardness than the core hardness. Selective hardening of the ball race is obtained by applying energy to the surface of the ball race by induction heating, an electron beam or a laser beam to austenitize a surface layer which is rapidly cooled for hardening.

Microstructural effects of splat cooling a high-speed steel

Abstract: A metallographic investigation of splat-cooled M1 high-speed steel is described. Apart from general microstructural refinement, the splat-cooling process retains in solid solution much greater amounts of the alloying elements than conventional quenching processes. This stabilizes the austenite to such an extent that the M s temperature is depressed close to or below ambient temperature and no martensite is formed. The morphology of the matrix phases, δ-ferrite and austenite, is shown to depend critically upon the specimen thickness and hence local cooling conditions. Owing to the high solute supersaturation, subsequent tempering of the splat-cooled steel leads to precipitation of the vanadium-rich carbide MC on 100 planes in the δ-ferrite, in sharp contrast to the molybdenum-rich M2C carbide which is normally responsible for secondary hardening in conventionally processed high-speed steels. In the splat-cooled steel, M2C appears only at the solute-rich interdendritic boundaries. Sintering studies ...

Fracture and fatigue in M-50 and 18-4-1 high speed steels

Abstract: The plane strain fracture toughness values,K Ic , the fatigue crack growth rates and the tensile properties of M-50 and 18-4-1 high speed steels have been measured as a function of tempering temperature. The M-50 was a vacuum arc remelted grade (VIM-VAR) and the 18-4-1 an electroslag grade, and both are used in mainshaft gas turbine bearings. At the usual hardness for bearings, Rockwell C 62, 18-4-1 exhibited a slightly higher fracture toughness (21 MPa·m1/2) than M-50 (18 MPa·m1/2). The fatigue crack growth rates were very similar, and in the slow growth region followed the usual power law,dC/dN=(ΔK) m withm=3 to 4. The crack propagation rates were still significant at values as low as ΔK=5 MPa·m1/2. SEM studies of the fracture surfaces showed complex transgranular fracture paths for both steels. The tensile strengths and the elongations of M-50 were somewhat higher than the corresponding values for 18-4-1 but the yield strengths of the two steels were similar. The microstructures of these steels were markedly different, with M-50 exhibiting 2.6 vol pct undissolved carbides and the 18-4-1 showing 15.2 vol pct carbides, but the fatigue and fracture behaviors were similar.

Practical Metallurgy and Materials of Industry

Abstract: 1. Extracting Metals from Ores. 2. Casting Processes. 3. The Physical and Mechanical Properties of Metals. 4. The Crystal Structure of Metals and Phase Diagrams. 5. Identification and Selection of Iron Alloys. 6. The Manufacturing of Steel Products. 7. The Iron-Carbon Phase Diagram. 8. I-T/T-T-T Diagrams and Cooling Curves. 9. Heat Treating Equipment. 10. Annealing, Stress Relieving and Normalizing. 11. Hardening and Tempering of Steel. 12. Hardenability of Steels. 13. Welding Processes for Iron and Iron Alloys. 14. Identification of Nonferrous Metals. 15. Heat Treating of Nonferrous Metals. 16. Metallurgy of Welds: Nonferrous Metals. 17. Powder Metallurgy. 18. Precious Metal Processing. 19. Corrosion of Metals. 20. Nondestructive Testing. 21. Composite Materials. 22. Plastics and Elastomers. 23. Ceramic Materials. 24. Wood and Paper Products. 25. Adhesives and Industrial Lubricants and Gases. 26. Hardness Testing. 27. Failure Analysis. Appendix 1: Industrial Methods of Specimen Preparation. Appendix 2: Tables. Appendix 3: Self-Evaluation Answers. Glossary. Index.

Tempering of high-purity and commercially based steels containing 10 wt-% tungsten or 5 wt-% molybdenum

Abstract: The effect of similar atomic percent additions of tungsten and molybdenum on the tempering behaviour of high-purity ternary and commercially based medium-carbon steels has been examined. A high-purity steel containing 5.6 wh%Mo (3.3 at.-%) showed substantially higher secondary hardening than an alloy with 9.9 wt-%W (3.2 at.-%). Marked differences in hardness were not observed when additions of 3 wt-%Cr and 0.3 wt-%V, frequently found in die steels, were present. Reasons for this behaviour are discussed.On tempering the ternary steels the alloy carbide precipitation sequence was M2C → M6C and the close association of M2C and M6C in some regions of the steel containing molybdenum suggested that an in situ transformation might be occurring. In the commercially based steels the presence of chromium led additionally to the formation of relatively small amounts of M23C6 during the early stages of tempering. Analytical electron microscopy has been used to examine the composition of the M2C and M6C carbid...

Production of hardened piston ring made of steel

Abstract: PURPOSE: To easily produce a hardened piston ring made of steel of high hardness under superior dimensional and shape accuracy by heating a wirelike or striplike steel material and forming the same to a piston ring shape, then immediately hardening and tempering, the same. CONSTITUTION: A steel material being a blank material for piston rings is formed to a strip shape by drawing or roll forming and is heated to a temperature higher than the hardening temperature of this steel material. Next, it is put in a ring forming machine and is thereby formed to a piston ring shape. It is mechanically restrained and held in that state and is then quenched by oil, water, etc. whereby it is hardened. Since the material is formed to a piston ring shape and is quenched and hardened in that state without being removed from the device, it is hardened without giving any change in its shape and after it has been subjected to necessary working, it is fixed in jigs and is hardened. These obviate the production of any strain in shape owing to hardening and tempering and enable the hardened piston ring of steel of superior dimensional and shape accuracy to be produced. COPYRIGHT: (C)1981,JPO&Japio

Low Mn alloy steel for cryogenic service and method of preparation

Abstract: A ferritic cryogenic steel which has a relatively low (about 4-6%) manganese content and which has been made suitable for use at cryogenic temperatures by a thermal cycling treatment followed by a final tempering The steel includes 4-6% manganese, 002-006% carbon, 01-04% molybdenum and 0-3% nickel

Producing glass sheets having improved break patterns

Abstract: Producing glass sheets having improved break patterns over those produced either by typical heat strengthening procedures or by heating to a tempering temperature followed by natural cooling.

Process for the thermal treatment of aluminium - copper - magnesium - silicon alloys

Abstract: A process for the thermal treatment of wrought products made of an aluminum alloy of the 2000 series comprising by weight from 3.5 to 5% copper, from 0.2 to 1% magnesium, from 0.25 to 1.2% silicon, with a Si to Mg ratio greater than 0.8 comprising solution heat treatment, quenching treatment, ageing and tempering wherein the tempering treatment comprises at least two stages: (1) a main tempering treatment at a temperature higher than 225° C. and lower than 280° C. lasting between 6 seconds and 60 minutes and (2) a complementary tempering treatment at a temperature between 120° and 175° C. lasting between 4 and 192 hours. This process allows the compromise between the mechanical tensile characteristics and the resistance to intercrystalline and stress corrosion to be improved.

Some cases of different structural sensitivity of impact strength and fracture toughness

Abstract: Two cases of practical importance are observed of a breakdown in the symbatic correlation of impact strength and fracture toughness concerning the influence of overheating before hardening and also of irreversible temper brittleness of high-strength steels. In both cases the breakdown in the correlation is related to differences in the structural and mechanical situations occurring at the opening of stress concentrators differing in the sharpness of the notch and, consequently, in the dimensions of the zones of plastic deformation and the degree of triaxiality of the stressed and deformed state. Differences in the behavior of KIc anda n with overheating of high-strength low-temperature-tempered steels reflect the fact of a decrease in the work for origin of a brittle crack with an increase in austenitic grain size. In some steels this is the overwhelming portion of the work for failure of Mesnager samples. A necessary condition for the appearance of temper brittleness in fracture-toughness tests is a shift in the temperature range of its appearance of the mechanism of propagation of a crack from intragranular to intergranular. The above-described breakdown in the correlation between impact strength and fracture toughness will appare: γ be characteristic of other cases of treatment of materials leading to the appearance of appreciable differe:; s in the strength of layers near the boundary and of the body of the grain with a low level of material plasticity. The cases considered are an indication of the incorrectness of choosing fracture toughness as a universal index of resistance to brittle failure of alloys without taking into consideration their structural state and the specific elastoplastic situation in mechanical tests determining the conditions of formation and growth of a brittle crack. With an increase in tempering temperature and also with a drop in carbon content in the steel there is a decrease in the share of the work for origin of a crack ina n. In connection with this for high-temperature-tempered and low-carbon steels fracture toughness becomes a more representative characteristic of resistance to brittle failure of alloys and maintains a symbatic correlation with impact strength.

Chromizing of steels by gaseous method

Abstract: The object of the present invention is an improvement in chromizing methods, constituted by a method of chromizing steels to a depth e greater than 30 microns, usable for steels with a carbon content of at least 02%, especially for steels for construction work and steels for tools, characterized by the combination of three successive treatments, the first of these three treatments consisting of an ionic nitriding of a surface layer between 100 and 350 microns thick, this ionic nitriding being realized in an atmosphere constituted by a mixture of nitrogen and hydrogen, at a temperature of between 450° C and 650° C, for between 5 and 40 hours, so as to obtain between 15% and 25% nitrogen in the nitrided layer, the second of these treatments consisting of a chromizing by gaseous method forming chromium carbides, lasting between 5 and 30 hours, and realized at temperatures of between 850° C and 1,100° C, the third of these three treatments being a thermal treatment comprising a quenching in oil of the chromized piece followed by a tempering at a temperature of between 600° C and 650° C, lasting between 30 minutes and 10 hours, depending on the size of the piece treated

Process for the thermal treatment of Fe-Co-Cr alloys for permanent magnets

Abstract: A process for the thermal treatment of an Fe-Co-Cr alloy for a permanent magnet, constituted by weight of 10 to 40% of Cr, from 0 to 10% of one or more elements of the group Al, Nb, Ta, W, Mo, V, Ti, Si and Cu, the remainder being iron. The process comprises homogenization treatment at between 1200° and 1400° C., an annealing treatment and one or more tempering treatments at between 500° and 600° C.; the annealing treatment comprises two stages, the first at between 630° and 670° for 5 to 30 minutes, the second, without a return to ambient temperature, at a temperature of from 40° to 70° C. below the previous stage for at least 10 minutes. Isotropic or anisotropic magnets whose hysteresis curve exhibits a better rectangular shape are thus obtained.

High tensile structural steel having superior strain relief treating embrittlement resistance

Abstract: PURPOSE: To prevent a toughness deterioration of a base metal even in slow cooling after tempering and in strain relief treatment after welding by composing a high tensile structural steel of respectively specified amts. of C, Si, Mn, Cr, Mo, etc., a restricted total amt. of P, Sn, Sb and As as temper embrittlement elements, and the balance Fe, etc. CONSTITUTION: A high tensile structural steel is composed of C; 0.05W0.2%, Si≤ 0.1%, Mn; 0.2W1%, Cr; 0.3W2%, Mo; 0.2W0.9%, Ni; 2.5W6%, V≤0.15%, Al≤0.03%, N; 0.003W0.01%, Cu≤0.8%, Nb≤0.1%, Ti≤0.05%, P, Sn, Sb and As as temper embrittlement elements 0.025% or less in total, and the balance Fe and inevitable impurities. This steel makes conventional quenching after tempering unnecessary and can be subjected to strain relief treatment, so the design standard of a welded structure is considerably improved, and drawbacks with respect to a conventional high tensile structural steel are overcome. COPYRIGHT: (C)1980,JPO&Japio

Production of high toughness bainite high tension steel plate having excellent weldability

Abstract: PURPOSE: To produce steel plate for line pipe in cold regions having excellent strength, toughness and weldability by a method wherein steel having specified compositions is heated at a low temperature and rolled under specified rolling condition to form steel having bainite structure, and the said steel having the above properties after the above rolling treatment or after tempering treatment. CONSTITUTION: Steel materials contain C 0.003W0.035%, Si<0.4%, Mn 1.4W 2.0%, S<0.006%, Al 0.05W0.08%, Nb 0.01W0.08%, Ti 0.005W0.025%, B 0.0008W0.0018% and N 0.001W0.005%, and have a composition within the range 0≤Ti%-3.4×N%≤0.01. Further, the steel materials may contain REM 0.001W 0.03% and/or Ca 0.0005W0.005% and/or a specified amount of one or more of V, Cr, Mo and Ni, and have a composition within the range 1≤REM/S≤10 and Mn+Cr+2Mo≤2.5. The steel materials are heated to below 1150°C. By subsequent rolling, cumulative rolling reduction at a temperature below 900°C is above 60%, and finishing temperarure is from 550 to below 700°C. COPYRIGHT: (C)1980,JPO&Japio

Effect of isothermal tempering on acoustic emission during ductile fracture of low-alloy steel

Abstract: EN30A, a low-alloy steel, has been tested under tensile load in the quenched condition and after tempering at 650°C for times up to 10080 min in order to investigate systematically the effect of strength and microstructure on acoustic emission. Measurements of the mechanical properties were combined with metallographic studies and extensive fractography in an attempt to determine the mechanisms of deformation and fracture which can generate detectable acoustic emission. The results are consistent with the fracture and/or decohesion of inclusions during both pre- and post-yield loading and fast shear fracture immediately before final fracture, being sources of detectable emission for all heat-treated conditions. In addition, the quenched specimen generated copious emission during plastic deformation, which increased in intensity as final fracture was approached, and this appeared to be linked to the onset of a localized shear mode of deformation and fracture. Material tempered for short times exhib...

Tempering of a beta quenched Zr-1.9 wt % Cr alloy

Abstract: The precipitation of ZrCr2, which is a Laves phase with the C15 structure, in a supersaturated, martensitic matrix has been studied in a Zr-1.9 wt % Cr alloy using transmission electron microscopy. The nucleation, morphology and distribution of the precipitates has been examined at different tempering temperatures. The crystallographic orientation relationship between the α-Zr(Cr) and the ZrCr2 phases has been established and the precipitate habit plane determined.

Hardening method for surface of turbine blade

Abstract: PURPOSE: To prevent the erosion due to drain and the stress corrosion cracking of turbine blade, by flame hardening the surface of an erosion-preventive layer of a turbine blade, then by tempering said surface at a specific temp., and by shot peening it. CONSTITUTION: Erosion-preventive layer 13 in the range of L 2 length from the steam inlet side top end, which is liable to be eroded by drain, of a turbine blade 7 is flame hardened at 1,000W1,600°C temp. using a heating burner A. Then it is tempered at 500W600°C temp. for about 3hr for the purpose of the regulation of hardness and uniformalization of the distribution of carbide. The erosion-preventive layer has 600W700 Vickers hardness. Finally compressive stress is given to the surface by shot peening. COPYRIGHT: (C)1981,JPO&Japio

Influence of heat-affected-zone refinement on high-temperature crack growth in thick-section O·5Cr–O·5Mo–O·25V steel weldments

Abstract: Crack-growth tests have been made at 565°C on thick-section butt welds produced to assess the effect of manual metal arc process parameters on heat-affected-zone (HAZ) refinement in O·5Cr–0·5Mo–0·25 V steel. It was confirmed that the increased degree of HAZ refinement achieved by weld-procedure control resulted in improved resistance to crack propagation. It was also possible to relate instantaneous rates of crack growth to the local microstructures in the HAZ at the tip of the crack by potential-drop measurements andpost-test metallography. The behaviour of a weld subject to post-weld renormalizing and tempering, as an alternative means of achieving microstructural refinement, was also examined. In this case it wasfound that crack initiation was delayed and the overall growth rate reduced relative to the welded and tempered structures. However, crack growth was accompanied by low displacements, similar to those for an as-welded untempered specimen. These were attributed to the concentration of st...

Permanent magnetic properties of Fe-Cr-Co-Mo alloy

Abstract: Changes in microstructure, hardness and magnetic properties of the Fe-Cr-Co-Mo alloy were examined under the influence of isothermal tempering and with optimum heat-treatment. At a temperature of 1350 K the alloy shows considerable increase in hardness which is connected with the separation of the phase σ. The temperature of 1350 K should be recognized as the lower limit of hot-working. The alloy obtains optimum magnetic properties after multistage ageing between 870 and 810 K with previous thermomagnetic treatment at 910 K. The greatest changes in magnetic properties take place in the first two stages of ageing (870 K, 2 h; and 850 K, 2 h), whereas in further stages only (BH)max increases. During tempering, in the 870 to 810 K range, the microstructure does not change. A slight overstepping of temperature to 910 K during thermomagnetic treatment causes precipitation of the σ phase, growth occurring at multistage ageing.

Detection of Ni–Sn phases in temper embrittled Ni–Cr steels

Abstract: A GENERIC problem in ferritic steels is the reduction in the grain boundary cohesive strength produced by the segregation of trace impurities of Groups IVB and VB of the Periodic Table. Such segregation results in a deterioration in mechanical properties produced, for example, by reversible temper embittlement1 and it also promotes enhanced susceptibility to a range of environmental enhanced cracking processes2–4. It has long been recognised that the embrittlement of steels is enhanced by the presence of certain alloy elements, that is Ni, Cr or Mn, and measurements using Auger electron spectroscopy (AES)5–7 have demonstrated that specific combinations of alloy and impurity elements cosegregate to the boundaries, for example, Ni+Sn, Ni+Sb, Cr+P. These observations can be rationalised using Guttmann's phenomenological theory8 which attributes the increased driving force for segregation to a positive interaction between alloy and impurity solute atoms. Previous investigations1,9–11 have failed to detect the presence of any three dimensional phase using conventional transmission electron microscopy and it is generally considered that the alloy element–trace impurity atoms form two dimensional phases at the boundaries. The only direct evidence for such alloy–impurity interactions and phase formation has been obtained using Mossbauer spectroscopy12,13 but it is not possible using this technique to draw any positive conclusions regarding the location of such compounds. Measurements on a pure Fe-3.5 wt% Ni alloy doped with 119Sn12 showed that Ni atoms associate preferentially with Sn atoms during tempering at 650°C and the marked change in the spectrum observed on ageing at 500°C, within the embrittling temperature range, is consistent with the formation of a Ni–Sn compound (Ni3Sn2 or Ni3Sn4). We report here some initial results from a detailed electron microscopic analysis of the microstructures of a commercial (En30A) Ni–Cr steel and a special laboratory made steel (BE 10) having similar Ni and Cr contents, but doped specifically with tin. The composition of the two steels is presented in Table 1. Both steels were oil quenched from 950°C, tempered for 1 h at 650°C and aged at 500°C for times up to 5,000 h. Carbon extraction replicas, taken from polished surfaces were examined in a Philips EM400 microscope fitted with an energy dispersive X-ray system. Selected individual precipitate particles were analysed using convergent beam microdiffraction and X-ray analysis. Attention was focused on the particles which delineated the lath packet and previous austenite boundaries, that is, high-angle boundaries.

Heat treatment of high strength nodular cast iron

Abstract: PURPOSE:To prevent the lowering of strength due to the rise of temperature by the procedure in which a nodular cast iron containing Ni is subjected to a hardening treatment ranging the austenitic zone to the Ms point, followed by a subzero treatment and a tempering treatment. CONSTITUTION:The molten cast iron, containing 1.7 to 3.8 wt% C, 0.8 to 3 wt% Si, and 0.3 to 2 wt% Mn, is added 6 to 12 wt% Ni, and then graphite is spheroidized by using a Fe-Si-Mg spheroidizing agent. Very shortly thereafter, the molten cast iron is inoculated with Fe-Si. The molten nodular graphite cast iron is cast into a sedired mold and then after solidification is taken out of the mold. Then, it is hardened at a temperature ranging the austenitic zone to the Ma point and subjected soon to a subzero treatment with liquid nitrogen. After being allowed to stand in the air, it is then heated at 150 to 400 deg.C for 30 min, cooled by air, and tempered. The treatment gives the cast iron a tensile strength of more than 140 kg/mm even at high temperature of 300 deg.C.

Effects of annealing of elongated nylon 6

Abstract: The shrinkage of extended nylon 6 was studied by the methods of isothermal annealing and tempering. The shrinkage was discussed in comparison with that of amorphous polymer solids in the glass transition region. It was found that the annealing below the melting temperature of crystalline polymer solids showed two effects: one was the shrinkage of samples tracing the quasi-equilibrium states and the other was the relaxation process stabilizing the strain. The rate of shrinkage of nylon 6 during tempering became large at the two temperature ranges, i. e., one was the glass transition region and the other the neighborhood of elongation temperatures above 100°C. Because of the effect of relaxation stabilizing the strain, the rate of shrinkage during tempering decreased at intermediate temperatures between the two temperature ranges. A stabilizing effect also was found in the fact that γ type crystallites were stabilized during tempering.

Effects of substructure on the mechanical properties of Fe-Ni-Co-C alloys

Abstract: The effect of transformation substructure (lath or twinned plates) and its subsequent modification of carbide distribution during tempering on the mechanical properties was investigated in Fe-Ni-Co alloys with and without 0.1 pct carbon. The morphology and substructure of carbon free and 0.1 pct carbon Fe-Ni-Co martensites do not have a significant effect on fracture toughness. The transformation substructure by itself does not control the deformation mode of these martensites. Based on a previously suggested model on factors affecting the mode of deformation, the need for substructure control to maintain desirable mechanical properties of low alloy high strength steels, is again emphasized.

Touch, wear resistant steel

Abstract: PURPOSE: To provide tough, wear resistant steel excellent in wear resistance and toughness, and useful for making construction machine etc., by heating at a specific rate a steel contg. specific amounts of C, Si, Mn, and Cr to harden it, then by tempering it. CONSTITUTION: A steel comprising 0.5W1.2% C, 0.7W1.7% Si, 0.3W1.0% Mn, 0.3W2.0% Cr, the balance being Fe, is heated at a rate of 10W200°C/min. to a temp. 750W950°C, and is kept at the temp. for max. 30 min. to harden the steel, then it is tempered to produce tough, wear resistant steel having martensite and carbide in its structure. COPYRIGHT: (C)1980,JPO&Japio

Cast steel for ultra low temperature

Abstract: PURPOSE:To enhance the low temp. strength of a 9% Ni steel as a cast steel for ultra low temp. use without deteriorating the low temp. toughness by adding a specific amt. of Mo. CONSTITUTION:A cast steel for ultra low temp. use consisting of C; 0.02-0.15%, Si<0.40%, Mn; 0.3-1.0%, Ni; 7.5-10.0%, Mo; 0.1-0.5%, sol Al; 0.01-0.06%, P.S<0.02% and the balance Fe. By adding Mo; 0.1-0.5% to a 9% Ni cast steel for ultra low temp. use used as support blocks supporting a liquefied natural gas storage tank and pumps, valves flanges, casings, etc. used at ultra low temp., the hardenability of the steel is improved and the structure after tempering is converted into of fine uniform martensite, whereby the low temp. strength is increased without deteriorating the low temp. toughness and the occurence of temper brittleness is prevented during cooling after tempering.