GB2073249A - Ferrite Free Precipitation Hardenable Stainless Steel - Google Patents

Abstract

An age-hardenable stainless steel having less than 5% by volume ferrite in all treatment conditions and improved ductility, consisting of, in weight percent, from 0.07% to 0.12% carbon, 0.20% to 3.0% manganese, 0.07% maximum phosphorus, 0.15% maximum sulfur, 2.0% maximum silicon, 15.5% to 17.5% chromium, 6.0% to 9.0% nickel, 0.95% to 2.50% aluminum, 0.005% to 0.20% nitrogen, 0.50% maximum molybdenum, 0.75% maximum copper, up to 0.12% titanium when nitrogen does not exceed 0.035% up to 0.05% cerium when nitrogen is greater than 0.035%, up to 0.07% boron, and balance essentially iron.

Description

SPECIFICATION Ferrite-free Precipitation Hardenable Stainless Steel The present invention relates to an age-hardenable stainless steel which is single-phased in its various conditions, and provides strength, hardness and ductility superior to that of nominal 1 7-7 PH stainless steels which ordinarily contain up to about 15% delta ferrite in all conditions. Although not so limited, the steel of this invention has particular utility as spring-temper material.

The steel of the present invention exhibits good hot workability, capability of cold-drawing reduction up to 90% without intermediate annealing, high proportional or elastic strength, and a uniform martensite transformation temperature below ambient temperatures.

As is generally known in the art, the heat treatment of 1 7-7 PH steels requires the following steps: Solution heat treatment (anneal) at 10380+140e (for bar or wire) or at 10660+1 40C (for sheet or strip) for 30 minutes minimum to obtain Condition A.

Austenite conditioning at 760 i 14 C for 90 minutes.

Transformation of austenite to martensite by cooling within 1 hour to 100--15.60C for 30 minutes minimum to obtain Condition T.

Precipitation hardening by heating to 565.60f5.60C (10500F) for 90 minutes, followed by air cooling to room temperature, to obtain Condition TH 1050.

Higher mechanical properties are obtained by the alternative treatment of Condition A material as follows: Austenite conditioning at 9540+9 OC (1 750"F) for 10 minutes, and air cooling, to obtain Condition A 1 750.

Transformation of austenite to martensite by cooling within 1 hour to 73.30+5.50C(1 000F) and holding for 8 hours, to obtain Condition R 100.

Precipitation hardening by heating to 5100f5.50C (9500F) for 60 minutes, followed by air cooling to room temperature, to obtain Condition RH 950.

The highest mechanical properties are obtained by the alternative treatment of Condition A material as follows: Cold reduction of at least 50% (at the mill), thereby transforming austenite to martensite, to obtain Condition C.

Precipitation hardening by heating to 482.20+5.50C (9000F) for 30 minutes (bar and wire) to 60 minutes (sheet and strip), followed by air cooling to room temperature, to obtain Condition CH 900.

The steel manufacturer ordinarily supplies 1 7-7 PH stainless steel in Condition A or Condition C in the forrn of sheet, strip, plate, bar or wire. The purchaser then ordinarily carries out the desired fabrication of the Condition A or Condition C material and conducts the above described treatments on the fabricated products to arrive at Condition TH 1050, Condition RH 950, or Condition OH 900.

SAE establishes the following composition, in weight percent, for 1 7-7 PH steel, in AMS 5528D (as revised 1-15-78): Carbon 0.09% maximum, manganese 1.00% maximum, silicon 1.00% maximum, phosphorus 0.040% maximum, sulfur 0.030% maximum, chromium 16.00% to 18.00%, nickel 6.50% to 7.75%, aluminum 0.75% to 1.50%, and balance iron.

Conventional 1 7-7 PH stainless steel produced in accordance with the above specifications exhibits a number of problems, including strain-cracking during cold drawing, transformation of Condition A material at about 1800 (00F), hydrogen embrittlement of the delta ferrite constituent during copper plating (applied as a lubricant for wire drawing), non-uniform properties in Condition C and Condition CH 900, and requirement of double or triple conversion treatments from 33 cm ingots to 10 cm square billets.

The above problems arise primarily from the presence of about 10% to about 1 5% delta ferrite which remains unchanged in all the various conditions of treatment of the steel. The delta ferrite is the locus of fracture during mechanical deformation. In the cold drawing of bar and wire sections longitudinal splitting develops and propagates through regions of delta ferrite. Moreover, since delta ferrite is not strengthened by cold working or heat treatment to the extent developed in the austenite matrix, the delta ferrite content prevents attainment of the maximum potential mechanical properties of 17-7 PH.

The presence of about 10% to 15% ferrite in conventional 1 7-7 PH steel in the solution annealed condition is a direct result of the lack of metallurgical stability. The aluminum content of 17-7 PH steel lowers the minimum temperature at which delta ferrite is a stable phase therein. Thus, heating and mechanical hot working do not induce sufficient chemical homogenization to produce a fully austenitic structure, as is obtained in AISI Type 301 (containing very little aluminum).

Despite the above problems, the presence of delta ferrite in amounts up to about 15% has been tolerated in conventional 1 7-7 PH steels, because it has been considered necessary to specify commercial ranges broad enough to permit operation by steel producers within practicable limits.

Earlier patents disclosing 17-7 PH steels include United States Patents 2,505,763; 2,505,764; 2,506,558 and 2,553,707, all issued to George N. Goller.

To the best of applicant's knowledge, prior art attempts to modify conventional commercial ranges for 1 7-7 PH steels have either been unsuccessful in eliminating delta ferrite or have resulted in adverse effects on other desired properties of the steel.

United States Patent 3,253,908 to Harry Tanczyn discloses a precipitation hardenable steel, the broad ranges of which consist essentially of 9.00% eo 20.00% chromium, 2.50% to 8.00% nickel, 0.70% to 2.50% aluminum, 1% to 5% molybdenum, with the sum of the chromium and molybdenum contents 14% to 21%, 0.10% to 0.40% nitrogen, 0.12% maximum carbon,8.00% maximum manganese, with manganese inversely proportioned to the nickel content,2.00% maximum silicon, 0.050% maximum phosphorus, 0.050% maximum sulfur, and remainder substantially iron.Nitrogen and molybdenum are essential elements in the steel of this patent, but despite the strong austenite-forming potential of nitrogen, the steel was indicated to contain delta ferrite in amounts not exceeding about 10% by volume. The high nitrogen content constitutes a problem since it reacts with the aluminum added for precipitation hardening, and the resulting aluminum nitride compounds are occluded in the solidifed metal with resultant adverse effect on ductility and cold formability.

The essential presence of molybedenum constitutes a further problem, since it is an even stronger ferrite former than chromium.

United States Patent 3,071,460, to Harry Tanczyn, also discloses a precipitation-hardenable chromium-nickel-aluminum stainless steel having a purposeful nitrogen addition of 0.10% to 0.40%, in which carbon is restricted to a critically low level of 0.03% maximum, the steel of this patent containing no molybdenum.

United States Patent 3,117,861 discloses a precipication-hardenable chromium-nickel-aluminum stainless steel containing purposeful additions of at least one of titanium, zirconium and uranium in order to react with nitrogen dissolved in the steel, thereby preventing objectionable occlusions of aluminum nitrides. This modification was for the primary purpose of avoiding porosity in welding along the junction of the weld bead and base metal.

An article by W. C. Clarke, Jr. and H. W. Garvin, entitled "Effect of Composition and Section Size on Mechanical Properties of Some Precipitation I Hardening Stainless Steels", ASTM Special Technical Publication No.369(1965), pages 151-158, deals with the problem of poor transverse ductility in large sections and in the short transverse direction in thin sections, in alloys such as 17-7 PH and PH 1 5-7 Mo. The presence of substantial amounts of delta ferrite and precipitates at the rims of the delta ferrite pools are stated to be responsible for this problem. Studies have determined that heat treatments of any type were not effective in improving transverse ductility, and vacuum remelting similarly has no effect.

A modified composition is described, in which aluminum is stili used as the hardening agent, designated as PH 1 3-8 Mo, which is free of delta ferrite and hence exhibits good transverse ductility, particularly when double vacuum melted by induction and remelt procedures. The modification designated as PH 1 3-8 Mo includes a substantial reduction in the carbon content to a level of about 0.025%, as well as a reduction in the chromium content to 13%, an increase in the nickel content to 8% and addition of about 2.25% molybdenum, as compared to 1 7-7 PH. The modified alloy is martensitic in the solution-treated form and can be precipitation hardened by a simple low temperature treatment.

In discussing 1 7-7 PH and PH 1 5-7 Mo, the authors state: "At the chromium levels involved, necessary composition balances result in a secondary structural phase which consists of fairly substantial amounts of delta ferrite. This phase may constitute as much as 12 z to 30 percent of the structure in these two alloys. Delta ferrite has been observed to detract from transverse ductility in other martensitic stainless steels, and it does so in these alloys." The above article, although published in 1965, constitutes a current description of the state of the prior art with respect to alloys of this type.

It is evident that the solution arrived at by prior workers involved primarily a reduction in carbon and chromium contents, in order to eliminate the existence of delta ferrite. These modifications result in the loss of a stably-austenitic structure and a sharp increase in the minimum hardness attainable in the alloy in various conditions of treatment. The cold-formabiiity of the low carbon and low chromium modification is also limited.

It is therefore evident that there still exists a need for a precipitation-hardenable stainless steel which avoids the problems or disadvantages exhibited by prior art alloys of this type which contain aluminum as a hardening agent.

It is a principal object of the present invention to provide an age-hardenable chromium-nickelaluminum-iron stainless steel which exhibits less than 596 delta ferrite in all its conditions of treatment, which in solution-treated condition exhibits higher tensile ductility and lower tensile strength than conventional stainless steel, which exhibits no delta ferrite strain cracking in cold working operations, and which exhibits metallurgical stability at temperatures down to about-190O (-200F).

It is a further object of the invention to provide such a steel which possesses improved hot workability of ingots and billet sections and which is amenable to greater degrees of cold working without mechanical breaking than a conventional 17-7 PH ,and more specifically the capability of being cold drawn with a reduction of about 90% in cross section without intermediate annealing.

The above objectives are obtained in the steel of the present invention by a critical balancing of the carbon, manganese, silicon, chromium, nickel, aluminum, and nitrogen contents. Additionally, molybdenum must be restricted to residual levels not greater than 0.5%. Boron may be added optionally as an aid to carbide precipitation in the austenite conditioning heat treatment. Titanium and/or cerium may be added for control of aluminum nitride formation.

According to the invention there is provided an age-hardenable stainless steel which is substantially single-phased in all treatment conditions consisting essentially of, in weight percent 0.07% to 0.12% carbon, 0.20% to 3.0% manganese, 0.07% maximum phosphorus, 0.15% maximum sulfur, 2.0% maximum silicon, 15.5% to 1 7.5 % chromium, 6.0% to 9.0% nickel, 0.95% to 2.50% aluminum, 0.005% to 0.20% nitrogen, 0.50% maximum molybdenum, 0.75% maximum copper, up to 0.07% boron, up to 0.12% titanium when nitrogen does not exceed 0.035%, up to 0.05% cerium when nitrogen is greater than 0.035%, and balance essentially iron.

Carbon is essential within the range set forth above for its strong austenite forming and austenite stabilizing potential.

Manganese is needed for stabilization of pre-established austenitic microstructures.

Silicon is limited to a maximum of 2.0%, and preferably to a maximum of 1.0%, since it is a ferrite former.

Chromium is of course essential for corrosion resistance. It is limited to 15.5% to 17.5%, and preferably 16.0% to 17.0%, since it is a ferrite former.

Nickel is essential for its austenite forming potential and as a precipitation-hardening agent. it is balanced with chromium within the range set forth above, and preferably from 6.5% to 8.0%.

Aluminum is essential as a precipitation-hardening agent. With less than 0.95% aluminum the volume-fraction of nickel-aluminum compounds available for interference-hardening is not sufficient to develop a minimum hardness of Rockwell C38 in the TH 1050 Condition or Rockwell C42 in the RH 950 Condition. Since aluminum is a ferrite former it is limited to a maximum of 2.50%.

Nitrogen is needed for its strong austenite forming potential and is preferably within the range of 0.02% to 0.08%.

Aluminum is more effective than copper as a precipitation-hardening agent because copper has a density 3.3 times greater than aluminum. Thus 1.0% by weight aluminum provides as many atoms as 3.3% by weight copper. Since the solubility limit of copper in iron-chromium-nickel alloys is about 4%, and since "free" copper affects hot malleability very adversely, the volume-fraction of nickel-copper compounds available for interference-hardening is quite limited, as compared to aluminium. Copper is also much more expensive than aluminum.

A preferred composition consists essentially of, in weight percent, 0.07 to 0.09% carbon, 1.0% maximum manganese, 0.04% maximum phosphorus, 0.025% maximum sulfur, 1.0% maximum silicon, 16.0% to 17.0% chromium, 6.5% to 8.00% nickel, 1.05% to 1.75% aluminum, 0.02% to 0.08% nitrogen, 0.50% maximum molybdenum, 0.50% maximum copper, 0.12% maximum titanium with titanium about 3.5 times the nitrogen content when nitrogen does not exceed 0.035%, 0.001% to 0.05% boron, and remainder essentially iron.

A preferred composition wherein carbon is increased slightly, a purposeful nitrogen addition is made, and cerium is substituted in place of titanium in order to prevent formation of aluminum nitrides, consists essentially of, in weight percent, 0.07% to 0.12% carbon, 1.0% maximum manganese, 0.04% maximum phosphorus, 0.03% maximum sulfur, 1.0% maximum silicon, 16.5% to 17.5% chromium, 6.75 to 7.75% nickel, 1.05% to 1.75% aluminum, greater than 0.035% to 0.20% nitrogen, 0.50% maximum molybdenum, 0.50% maximum copper, 0.004% to 0.02% cerium, 0.001% to 0.05% boron, and remainder essentially iron.

In the first preferred embodiment above, optimum results are obtained by maintaining carbon at or near the maximum 0.09%, controlling manganese and silicon to the lowest practicable levels, maintaining nickel at about 7.25% minimum, restricting aluminum at or near 1.4%, controlling molybdenum to the lowest practicable level, maintaining nitrogen at or near 0.035%, and maintaining titanium at about 3.5 times the nitrogen content.

In conventional 17-7 PH steel a maximum of 0.030% sulfur is specified in order to minimize the influence of sulfide particles on the tendency for longitudinal splitting during cold working operations.

Since the steel of the present invention is essentially ferrite free, it is a feature of the present invention that sulfur may be increased up to about 0.15% both in the broad and preferred compositions in order to obtain good machinability.

In the preferred composition wherein the carbon and nitrogen levels are increased above the maxima permitted under current conventional specifications, titanium is omitted, and cerium is substituted in order to prevent formation of aluminum nitride compounds. Substantially all the aluminum is thus available to function as a precipitation-hardening agent. It has been found that it is not necessary to add cerium in a stoichiometric ratio to the nitrogen, since cerium apparently forms a complex of some sort with nitrogen.

The properties of the steel of the present invention make it ideally suited for fabrication into products such as cold-formed and/or machined fasteners, spring temper products including helical springs, antenna rods, fishing rods, leaf springs, bellows, grinding balls, roller bearings and ball bearings.

The steel of the invention may be processed in accordance with the conventional treatments to obtain the various Conditions hereinabove described. More specifically, steel in Condition A may be solution annealed (to obtain Condition A 1750) by heating within the range of about 9550 to about 112000 for a time sufficient to effect solution of carbon and nitrogen compounds, followed by air, oil or water cooling to room temperature. as indicated previously the boron addition in the present steel aids the precipitation of carbides in the austenite conditioning.Transformation of the solution annealed material to alpha martensite can then be effected either by reheating within the range of about 6500 to about 9250C for a time up to about 3 hours and cooling to the range of about 100 to about 1 5.5 OC (Condition T), or cooling to about C and holding for time periods up to 8 hours (to obtain Condition R 1 00), or cold working of the solution annealed material with about 50% to 95% reduction in thickness or cross sectional area, to obtain Condition C. Finally, precipitation hardening is obtained by heating within the temperature range of about 4250 . about 5250C for periods of time up to 8 hours, followed by cooling to room temperature.

The improved ductility of the steel of the present invention offers a distinct advantage in that it can be supplied by the steel producer to fabricators in Condition C or Condition CH and fabricated into articles of ultimate use, which are subjected to precipitation hardening. As indicated previously, it is a further advantage that the steel of the invention can be cold reduced by as much as 90% to Condition C in a single cold reducing stage without intermediate anneals, a procedure which cannot be followed with conventional 1 7-7 PH steel.

As in known in the art, improvements in toughness, resistance to intergranular corrosion and fatigue strength are obtained by repeating the transformation treatment of Condition T or Condition R100 material prior to the precipitation hardening treatment.

Experimental heats within the preferred composition ranges have been produced, and these compositions are set forth in Table I. Mechanical properties in various conditions have been determined and are set forth in Table II, for Sample 3 (a preferred embodiment containing a purposeful nitrogen addition and cerium) and for a conventional 1 7-7 PH steel.

In Table Ill tensile and yield strengths are given for Samples 3, 4 and 5, which are embodiments containing a purposeful nitrogen addition and cerium, for cold drawing reductions ranging from 50% to 89% in the solution treated Condition A, in the transformed condition and in the precipitation-hardened condition.

The data of Tables II and 111 demonstrate that the steel of the present invention exhibits ultimate tensile and yield strengths comparable to those of conventional 1 7-7 PH steel along with higher elongation and reduction of area levels. The improvement in ductility of the steel of the invention is important during fabrication of cold drawn or cold rolled materials in order to minimize mechanical breakages therein. This permits production of spring temper characteristics in a broad range of wire and rod sizes.

Proportional limit or elastic strength levels are the basis for load carrying capacities for spring temper products. Hence the high levels ranging from about 1 70 to greater than 1 90 ksi in the steels of the invention provide a significant improvement over conventional 1 7-7 PH, which exhibits an elastic limit in the range of about 105 to 110 ksi.

As indicated above, the relatively high ductility of the present steel permits fabrication into articles of ultimate use in Condition C or even Condition CH. The soluble carbon level of the steel of the invention is such that the martensite transformation temperature is about 3400 (-200F), which insures against transformation during shipment in cold weather but does not constitute an unduly expensive transformation treatment for a fabricator if Condition A material is delivered and the fabricated products are subjected to treatments resulting in Condition Al 750 and Condition Tri 00.

The increase in tensile and yield strengths at various conditions of treatment are accompanied by corresponding increases in hardness. By way of example, Condition A material exhibits a Brinnel hardness of about 180, which increases to about 260 for Condition T material and to about 430 for Condition TH and Condition C.

To illustrate the critical balancing of elements which result in virtual elimination of delta ferrite in the steel of the invention, a comparison of a conventional 1 7-7 PH and steels of the invention is set forth in Table IV.

Additional heats of steels in accordance with the invention have been prepared and tested for mechanical properties in various treatment conditions. The compositions of these heats are set forth in Table V, and the mechanical properties thereof are summarized in Table VI, the samples having been converted from 5.1 cm thick billets to 2.5 4 mm thick strip.

Table I Composition-Weight Percent Sample C Mn P S Si Cr Ni Al N Ti Ce 1 0.095 0.54 0.020 0.014 0.34 16.89 7.28 1.24 0.022 0.060 2 0.10 0.50 0.022 0.023 0.34 16.82 7.31 1.21 0.031 0.067 3 0.076 0.87 0.012 0.014 0.60 17.08 7.34 1.07 0.10 - 0.005 4 0.081 0.85 0.012 0.019 0.50 16.72 7.26 1.02 0.11 5 0.083 1.07 0.012 0.013 0.81 17.07 7.20 1.19 0.095 Mo < 0.5%, Cu < 0.5%.

% delta ferrite : Samples 1 and 2 < 0.5%, Samples 3-5 = 0%.

Table II Mechanical Properties Prop. or U.T.S. 0.2% Y.S. Elastic S % Elong. % Reduction Sample Condition kg/mm kg/mm kg/mm in 5.1 cm of Area 3 C(hot rolled 6.35 mm 145.8 140.1 - 10.0 42.0 diameter rod coil; (average of 2) (average of 2) (average of 2) (average of 2) solution treated at 1038 C; cold drawn to 4.45 mmdiameter rod coil; straightened and cut to length) 3 CH (Condition C+ 178.0 174.4 137.1 10.5 38.9 454 C - 1 hr., air cool) 3 CH (Condition C+ 17.7 167.0 135.7 10.0 41.1 482 C - 1 hr, air cool) (average of 2) (average of 2) (average of 2) (average of 2) 3 CH (Condition C+ 166.8 154.2 121.6 12.2 39.4 510 C - 1 hr, air cool) (average of 2) (average of 2) (average of 2) (average of 2) (average of 2) Conventional C (treatment as above 149.0 146.9 - 4.0 35.0 17-7 PH in Example 3) Conventional CH (Condition C+ 185.6 181.4 75.9 4.0 35.0 17-7 PH 482 C - 1 hr, air cool) Table III Mechanical Properties-kg/mm Sample 3 Sample 4 Sample 5 Condition U.T.S. 0.2% Y.S. U.T.S. 0.2% Y.S. U.T.S. 0.2% Y.S.

A (6.35 mm diameter (100) 70.3 (32) 22.5 (101) 71.0 (30) 21.1 (103) 72.4 (32) 22.5 rod, picled) C (A cold drawn 50% (207) 145.5 (199) 139.9 (223) 156.8 (211) 148.3 (221) 155.4 (210) 147.6 reduction) CH (C + 482 C - 1 hr, (251) 176.5 (238) 167.3 (264) 185.6 (246) 172.9 (257) 180.7 (245) 172.2 air cooled) C' (A cold drawn- (238) 187.3 (227) 159.6 (241) 169.4 (229) 161.0 (240) 168.7 (231) 162.4 60% reduction) C'H (c'+ 482 C - 1 hr, (277) 194.7 (260) 182.8 (282) 198.2 (261) 183.5 (279) 196.1 (263) 184.9 air cooled) C" (A cold drawn - 89% (305) 214.4 (295) 207.4 (302) 212.3 (287) 201.8 (311) 218.6 (297) 208.8 reduction) C"H (C" + 482 C - 1 hr, (357) 251.0 (343) 241.1 (353) 248.2 (338) 237.6 (362) 254.5 (351) 246.8 air cooled) Table IV % Delta % Delta Ferrite Ferrite (10.2 cm (0.64 cm Sample C Mn Si Cr Ni Al N Billet) Rod) 17-7 PH 0.064 0.54 0.25 16.87 7.41 1.09 0.030 18 15.0 6* 0.087 0.92 0.39 16.90 7.60 1.26 0.029 7 1.7 7* 0.083 0.84 0.57 16.28 7.50 1.12 0.032 0 0 *Steels of the invention.

Table V Composition-Weight Percent Sample C Mn Si Cr Ni Al Mo Cu N Ti 8 0.091 0.91 0.38 16.92 7.51 1.12 0.21 0.32 0.031 0.046 9 0.082 0.89 0.33 17.10 7.36 1.19 0.14 0.11 0.032 0.064 10 0.087 0.80 0.32 17.01 7.57 1.17 0.15 0.12 0.024 0.062 Table VI Mechanical Properties U.T.S. 0.2% U.S. % Elongation Sample Condition kg/mm2 kg/mm2 in 5.1 cm 8 TH 1050 102.6 51.3 17 9 TH 1050 115.3 87.9 13 10 TH 1050 116.7 90.0 12 8 RH 950 132.9 116.7 11 9 RH 950 142.7 124.4 9 10 RH 950 146.9 130.8 11 8 Condition C* 151.1 147.6 6 9 Condition C* 150.4 141.3 6 10 Condition C* 147.6 142.7 10 8 Condition 187.7 182.1 3.0 CH-900 9 Condition 186.3 182.8 4.0 CH-900 10 Condition 185.6 184.2 2.0 CH-900 *Condition C=60% cold-reduction.

Claims (8)

Claims

1. An age-hardenable stainless steel containing less than 5% by volume ferrite in all treatment conditions consisting essentially of, in weight percent, from 0.07% to 0.12% carbon, 0.20% to 3.0% manganese, 0.07% maximum phosphorus, 0.1 5% maximum sulfur, 2.0% maximum silicon, 1 5.5% to 17.5% chromium, 6.0% to 9.0% nickel, 0.95% to 2.50% aluminum, 0.005% to 0.20% nitrogen, 0.50% maximum molybdenum, 0.75% maximum copper, up to 0.12% titanium when nitrogen does not exceed 0.035%, up to 0.05% cerium when nitrogen is greater than 0.035%, up to 0.07% boron, and balance essentially iron.

2. An age-hardenable stainless steel which is substantially single-phased in all treatment conditions, consisting essentially of, in weight percent, from 0.07% to 0.09% carbon, 1.0% maximum manganese, 0.04% maximum phosphorus, 0.025% maximum sulfur, 1.0% maximum sillcon, 16.0% to

17.0% chromium, 6.5% to 8.00% nickel, 1.05 % to 1.75 % aluminium, 0.02% to 0.08% nitrogen, 0.50% maximum molybdenum, 0.50% maximum coopar, 0.12% maximum titanlum with titanium about 3.5 times the nitrogen content when nitogen does not exceed 0.035%, 0.001% to 0.05% boron, and remainder essentially iron.

3. An age-hardenable stainless steel which is substantially single-phased in all treatment conditions, consisting essentially of, in weight percent, from 0.07% to 0.12% carbon, 1.0% maximum manganese, 0.04% maximum phosphorus, 0.03% maximum sulfur, 1.0% maximum silicon, 16.5% to 17.5% chrornium, 6.75% to 7.75% nickel, 1.05% to 1.75% aluminium, greater than 0.035% to 0.20% nitrogen, 0.50% maximum molybdenum, 0.50% maximum copper, 0.004% to 0.02% carium, 0.001% to 0.05% boron, and remainder essentially iron.

4. The steel claimed in Claim 2, wherein carbon is about 0.09%, nickel is at least about 7.25%, aluminum is about 1.4% and nitrogen is about 0.035%.

5. The steel claimed in Claim 1, consisting essentially of, in weight percent, from 0.07% to 0.09% carbon, 1.0% maximum manganese, 0.04% maximum phosphorus, 0.15% maximum sulfur, 1.0% maximum silicon, 16.0% to 17.0% chromlum, 6.5% to 8.0% nickel, 1.05%to 1.75% aluminium, 0.02% to 0.08% nitrogen, 0.50% maximum molybdenum, 0.50% maximum copper, 0.12% maximum titanium with titanium about 3.5 times the nitrogen content when nitrogen does not exceed 0.035%, 0.001% to 0.05% boron, and remainder essentially iron.

6. The steel claimed in Claim 1, consisting essentially of, in weight percent, from 0.07% to 0.12% carbon, 1.0% maximum manganese, 0.04% maximum phosphorus, 0.15% maximum sulfur, 1.0% maximum silicon, 16.5% to 17.5% chromium, 6.75% to 7.75% nickel, 1.05% to 1.75% aluminium, greater than 0.035% to 0.20% nitrogen, 0.50% maximum molybdenum, 0.50% maxirnum copper, 0.004% to 0.02% cerium, 0.001% to 0.05% boron, and remainder essentially iron.

7. The steel claimed in Claim 3, having an elongation in 5.1 cm of at least 10% after transformation to a substantially fully martensitic condition by cold reduction of at least 50% in cross sectional area.

8. An age-hardenable stainless steel substantially as hereinbefore particularly described.