US2795519A - Method of making corrosion resistant spring steel and product thereof - Google Patents

Description

June 11, 1957 2,795,519

T. ANGEL ETAL METHOD OF MAKING CORROSION RESISTANT SPRING STEEL. AND PRODUCT THEREOF Filed March 25, 1955 loa- INVENI ORS BY aging PAM JW PM ATTORNEY 5 P? ZWW METHOD OF MAKING CORROSION RESISTANT SPRING STEEL AND PRODUCT THEREOF Tryggve Angel and Axel Wilhelm Bernstein, Sandviken, Sweden, assignors to Sandvikens Jernverks Alrtiebolag, Sandviken, Sweden, a corporation of Sweden Application March 25, 1955, Serial No. 496,904 Claims priority, application Sweden March 27, 1954 6 Claims. (Cl. 148-12) The present invention relates to alloy steel springs and spring material having not only excellent spring properties such as high values of modulus of elasticity, ultimate strength and fatigue limit at low, normal and elevated temperatures but also good corrosion resistance. 7

Heretofore, in order to secure this combination of properties, highly alloyed and expensive steels containing alloying elements which interfere with the hot and cold working thereof have been used. Also carbon steel pro vided with permanent corrosion preventing coatings has been used. Such corrosion preventing coatings are however not always permanently effective and are costly and difiicult to apply and may cause hydrogen embrittlement of the steel.

For a long time the need has existed for a product having good spring properties made from the well known austenitic stainless steel compositions. Such compositions cannot however have their strength increasedby heat treatment in the usual Way because they retain their austenitic structure down to and below room temperature and therefore cannot have their strength increased by transformation of the soft austenite into hard martensite. A hardening, which is not generally desirable, and which is due to a certain transformation of austenite into martensite has been observed as a result of cold working of such austenitic alloys.

Earlier attempts to reduce the cross sectional area 25-60% by cold rolling of austenitic chromium nickel steels, followed by heat treating at a relatively low temperature, for example between 74-200 C. has given a certain increase in strength, which, however, is not sufficient when high strength and elastic properties, as for instance in spring steels for use in watches and instruments are required. Material treated in this way is not suitable for springs which have to withstand elevated temperatures, for example 300-500 C.

It has further been proposed to use certain metastable austenitic steels, which have been subjected to a very considerable decrease in cross sectional area preferably about 90%, by cold working followed by tempering. It is true that these steels, which are the subject of application Serial No. 260,856, filed on December 12, 1951, in the name of Axel Wilhelm Bernstein have given springs with excellent elastic properties, but a severe disadvantage thereof is the extreme degree of reduction, which gives rise to difliculties in their manufacture and necessitates very expensive equipment. With modern cold rolling mills it has not been possible to reduce a rough, relatively thick material as much as 90% and thus the limit of the thickness for the finished spring steel material has been restricted to about 0.5 mm.

We have found through extensive investigations that by choosing a suitable composition of the austenitic steel very excellent elastic properties may be obtained, even at a relatively low degree of reduction. The composition should be so chosen that the finished product after a reduction of the cross sectional area which may not be nited States Patent less than 50% and which should not exceed 75% should contain between 25 and 60%, preferably about 50% of martensite.

The rate of transformation from austenite to martensite is dependent partly on the stability of the austenite, i. e. its greater or' less tendency to transform and partly on the degree of deformation. cold working the quantity of martensite formed increases with decreasing stability of the austenite. Inversely, it can be said that for a certain stability of the austenite, the quantity of martensiteformed is increased with the degree of cold working. Any desired quantity of martensite may be obtained in the steel. The resulting strength will depend upon this transformation. bility of the austenite depends partly on the percentages of the alloying elements and partly onthe working temperature. fects upon the stability. By lowering the temperature the stability of the austenite is decreased. Thus the ,working temperature affords a further means of controlling the transformation.

Our experiments have shown that relatively insignificant changes in the percentages of the alloying elements, may cause a considerable change in the stability of the austenite. Thus many closely related materials, as to composition, show quite different elastic properties. On this account it is essential to characterize alloys suitable for, the invention on a different basis than the composition alone. We have therefore chosen to characterize the alloy with reference to its stability. The particular measure which we have chosen for the stability of the austenite, we will call Mdau analogical to the symbol Ms which latter represents the characteristic temperature for ordinary steels, where the austenite spontaneously starts to transform into martensite. Maao can be determined by a tensile test and can be defined as the characteristic temperature at which 50% of martensite is formed under tension at a true strain of 0.30 (30%). True strain is defined as in distinction from the conventional strain where lo=the original measured length of the test bar and l=the length after the elongation. We have further found that the effect of the most important alloying elements on the stability measured as Md30, may be determined by the following equation:

The terms within parenthesis signify the weight percentages of the various elements. Md30 should, according to our investigations, be chosen between 20 C. and 20 0., preferably between 10 C. and 10 C. in order to obtain the results characteristic for the invention. Through the above equation a further and closer definition of those austenitic alloys that are suitable for the invention is obtained. The said equation may of course be completed with further alloying elements.

According to the invention the specimens are made from alloys which after quench-annealing are austenitic and have such a stability that the deformationtemperature at which 50% martensite is formed from the austenite, after the material has been deformed in tension, corresponding to a true strain of 30%, will lie between 20 C. and 20 C., preferably between 10 C. and l0 C. and which-after quench-annealing and reduc- Patented June 11, 1957.

For a certain amount of.

The sta- Different alloying elements have different eftion of cross sectional area through cold-working more than 50% below 75% will contain between 30% and 60% of martensite, preferably between 40 and 60%.

Of great importance for the elastic properties is the amount of martensite in the finished specimens. .With a percentage of martensite outside the above mentioned range excellent elastic properties will not be obtained. As a rule the best results have been obtained when the percentages of austenite and martensite are about equal. The cold working is carried out in several steps without intermediate heat-treatment and the reduction of the cross sectional area may not be'less than 50%. Usually it should be between 50% and 70%, but in certain instances it has been necessary to proceed to 75%. The quenchannealing that precedes the cold working necessitates a heating to high temperature, usually within the interval 950-l200 C. with subsequent rapid'cooling. The carbides usually present in the austenitic steels after air cooling are thereby brought into solution, which gives the steels the properties necessary for the subsequent cold working.

The working temperature is another'factor, that "has great influence on the result. A lowering of this temperature below that normally used for cold Working will give a greater percentage of martensite at the samedegree of working or will require a lower'degree of working for obtaining the same percentage of martensite. It has been shown that such a lowering of the working temperature with a simultaneous decrease of the degree of reduction will not cause any change in the excellent elastic properties that are obtained with the austenitic alloys covered by the invention. This means great advantages from the point of view of manufacturing. By 1 keeping the degree of reduction lowit must not be less than 50%several reduction steps are saved and at the same time the dimensional range for finished spring ma terial may be considerably increased without going beyond the permissible roll pressure of the modern rolling mills.

The working temperature which is the easily measured temperature of the material immediately before entering the rolling mill may thus advantageously be below'rooru represent a true strain of 10, 20, 50, and 50% respectively. Fromthis it is plainly shown how the transformation proceeds with increasing degrees of deformation and also the considerable influence of the working temperature.

It should be pointed out, that the curves in the drawings represent actual experiments. Thus the alloys A and'B have the following compositions in percent of weight:

I A 1 B Carbon 0. 11 O. 06 Sllicon. 1.03 0. 39 Manganese. l. 15 0. 33 Chromium 17. 5 l8. 4 Nickel 8. 3 8. 6 Molybdenum 0. 88 0. 06 Nitrogen 0. 025 O. 024

The remainder iron with usual impurities.

Tensile strength "kg/B1134 65 r 65 Blast o kg./mm.n. 15 15 Martensite pereent 0 0 After cold tooling at room temperature of the quench- 1 annealedalloys in a plurality of steps without intermeditemperature. The temperature range between -10and 15 C. has proved to be particularly suitable for this purpose.

A further improvement in the elastic propertiesof the specimens, according to' the invention, may be obtained by tempering after cold working within the temperature range of 350550 C. fora time, which due to several factors such as the tempering temperature and the shape of the specimen may suitably be chosen within the range from a few minutes up to several days. 400-550 C. which has proved to be advantageous, the tempering time may advantageously be within the range from 2 to 8 hours. considerably longer timeshave in some instances proved to be advantageous, but areas a rule not necessary and would naturally increase the cost, which should be avoided.

In the accompanying drawings we have shown the relationship between the percentage of martensite, deformation temperature and true strain in the case of two diiferent austenitic alloys after quench-annealing.

In Fig. l are shown two alloys A and B, the former of which falls within the scope of the invention, while the latter obviously lies outside the scope of the invention. Both alloys have been subjected to a true strain of 30%, butonly alloy A has the defined stability at which martensite is formed within the temperature range from 20 to -20 C. For alloy A this characteristic temperature is about 5 C., while for alloy B the characteristic temperature is about 33 C.

Fig. 2 shows the percentage of martensite for alloy A as a function of the deformation temperature and asa function of'the truestrain. Thecurves 1, 2, 3, and-4 Within the range ate" annealing and-with a cross-sectional'area reduction of 60% the following properties were obtained.

A. further rise in tensile. strength and elastic limit was gained by a subsequent tempering at 425 C. for 4 hours.

Tensile strength kg/mum. 200 125 Elastic limit o' kg./mm.r-. 125 Martensite percent about 52 about 27 As earlier mentioned, the alloys should, after quenchannealing; be] fully austenitic, that is free from E-ferrite. This has a favourable influence on the corrosion resistance. The requirement for corrosion resistance means that the percentage of chromiumrnust not be less than 13%. In order that 6'-ferrite shall not be present, it is essential that the so-called ferrite forming elements, such as chromium, silicon and molybdenum are balanced against the austenite formingelernentssuch as nickel, manganese, carbon and nitrogen.

lthas been shown that alloys best suitable for the purpose in mind, should contain elements within following limits;v 0.07-0.20% j carbon; 0-0.l% nitrogen; 14-20% chromium; 712% nickel; 0.2-2% silicon; 0.44% manganese; 0-4% molybdenum and 040% cobalt or'0.07'- 0.20% carbon; 00.1% nitrogen; 14-20% chromium, 3'7% nickel; 0.2-2% silicon; 442% manganese; 04% molybdenum and 0-l0 cobalt. Besides one or more other: alloying elements may be added, suchas tungsten, titanium, columbium, tantalum, or aluminum in percentages up to 1%. Other elements in relatively small amounts may be present but the remainder consists substantially of iron.

Particularly suitable are alloys within the following closer limits: ODS-0.20% carbon; 0.0050.075% nitrogen; 16-18% chromium; 7-10% nickel; 0.8-l.5% silicon; 0.8-1.5% manganese; 0-2% molybdenum; 0-5% cobalt; 0-1% titanium; 0-1% columbium and/or tantalum; 0-2% tungsten; 0-1% vanadium and the remainder iron with usual impurities. A preferred range of analysis for steels at this type is: 0.10-0.15% carbon; 0.015-0.035% nitrogen; 17-18% chromium; 7.5-9% nickel; 0.9-1.20% silicon; 0.9-1.2% manganese and 0.5-1.5% molybdenum.

It is of great importance in obtaining excellent elastic or spring properties that the percentage of carbon shall be kept at a relatively high value for austenitic steels. It should not be lower than 0.07% and most suitably between 0.10 and 0.15%. Thus the increase in the mechanical strength due to a certain amount of martensite obtained in cold working is proportional to the carbon content. As an increase in the mechanical properties results in a corresponding increase in the spring properties it is easily understood that a proper choice of carbon content is of great importance.

The invention is in the first place concerned with springs of all types, for example watch springs, but other objects, in which excellent elastic properties are necessary, fall within the scope of the invention. There is no difficulty in producing springs according to the invention with thicknesses of up to at least 2 to 3 mm. with excellent spring properties and good corrosion resistance, at normal as well as at elevated temperatures for example 500 C. In comparison with carbon steel springs, the springs produced according to the invention, possess considerably better fatigue properties and are at the same time corrosion resistant.

We claim:

1. Method of making a steel object having good corrosion resistance and excellent spring properties which comprises quench-annealing a steel alloy from a temperature within the range from 950 C. to 1200 C., cold working the quench-annealed alloy in a plurality of steps without intermediate annealing with a cross-sectional area reduction within the range from to 75% at a temperature within the range from -20 C. to 20 C. to a martensite content of 40% to and tempering the resulting cold worked alloy within the temperature range from 350 C. to 550 C., said alloy having a composition within the limits 0.07% to 0.20% carbon, 0% to 0.1% nitrogen, 14% to 20% chromium, 7% to 12% nickel, 0.2% to 2% silicon, 0.4% to 2% manganese, 0% to 4% molybdenum, 0% to 10% cobalt and less than 1% of any one of the metals of the group consisting of tungsten, titanium, columbium, tantalum and aluminum with the remainder consisting substantially of iron, said alloy after said quench-annealing being fully austenitic and having such a stability that about 50% of martensite will be formed from the austenite when the quench-annealed alloy is deformed at a tension corresponding to a true tension of 30% at a temperature within the range from 20 C. to 20 C.

2. Method of making steel objects as defined in claim 1 in which the quench-annealed alloy has such a stability that about 50% of martensite will be formed from the austenite when said quench-annealed alloy is deformed at a tension corresponding to a true tension of 30% at a temperature within the range from -10 C. to 10 C.

3. Method of making steel objects as defined in claim 2 in which the alloy is tempered at a temperature within the range from 400 C. to 550 C.

4. Method of making steel objects as defined in claim 3 in which the cold working is carried out at a temperature within the range from '10 C. to 15 C.

5. Method of making steel objects as defined in claim 1 in which the alloy is cold worked to a reduction in cross-sectional area within the range from 50% to 6. A steel object having good corrosion resistance and excellent spring properties made by the method defined in claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,044,743

OTHER REFERENCES Transactions of the American Society for Metals, vol. 39, 1947. Pages 869-888.

Steel Processing, vol. XXXVII, Issue 1, January 1951, page 24.

Claims (1)

1. METHOD OF MAKING A STEEL OBJECT HAVING GOOD CORROSION RESISTANCE AND EXCELLENT SPRING PROPERTIES WHICH COMPRISES QUENCH-ANNEALING A STEEL ALLOY FROM A TEMPERATURE WITHIN THE RANGE FROM 950*C TO 1200*C., COLD WORKING THE QUENCH-ANNEALED ALLOY IN A PLUARLITY OF STEPS WITHOUT INTERMEDIATE ANNEALING WITH A CROSS-SECTIONAL AREA REDUCTION WITHIN THE RANGE FROM 50% TO 75% AT A TEMPERATURE WITHIN THE RANGE FROM -20*C. TO 20* C. TO A MARTENSITE CONTENT OF 40% TO 60% AND TEMPERSING THE RESULTING COLD WORKED ALLOY WITHIN THE TEMPERATURE RANGE FROM 350*C. TO 550*C., SAID ALLOY HAVING A COMPOSITION WITHIN THE LIMITS 0.07% TO0.20% CARBON, 0% TO 0.1% NITROGEN, 14% TO 20% CHROMIUM, 7% TO 12% NICKEL, 0.2% TO 2% SILICON, 0.4% TO 2% MANGANESE, 0% TO 4% MOLYBDENUM, 0% TO 10% COBALT AND LESS THAN 1% OF ANY ONE OF THE METALS OF THE GROUP CONSISTING OF TUNGSTEN, TITANIUM, COLUMBIUM, TANTALUM AND ALUMINUM WITH THE REMAINDER CONSISTING SUBSTANTIALLY OF IRON, SAID ALLOY AFTER SAID QUENCH-ANNEALING BEING FULLY AUSTENITIC AND HAVING SUCH A STABILITY THAT ABOUT 50% OF MARTENSITE WILL BE FORMED FROM THE AUSTENITE WHEN THE QUENCH-ANNEALED ALLOY IS DEFORMED AT A TENSION CORRESPONDING TO A TRUE TENSION OF 30% AT A TEMPERATURE WITHIN THE RANGE FROM -20*C. TO 20*C.

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