US2718482A - Heat treatment of vanadium steel to improve the creep strength thereof - Google Patents

Description

Sept. 20, 1955 E A sTlcHA Filed OCb. 3l, 1950 2,718,482 HEAT TREATMENT oF VANADIUM STEEL T0 IMPROVE THE CREEP STRENGTH THEREOF 3 Sheets-Sheea Sept. 20, 1955 E A, 5T|C||A 2,718,482

HEAT TREATMENT OF' VANADIUM STEEL TO IMPROVE THE CREEP STRENGTH THEREOF Filed Oct. 5l, 1950 5 Sheets-Sheet 2 Z 34 66 4 66 0./ [.0 Z j /0 Z J4 /w SePf- 20, 1955 E. A. sTlcHA HEAT TREATMENT OF VAN'ADIUM STEEL. TO IMPROVE THE CREEP STRENGTH THEREOE 3 Sheets-Sheet 3 Filed Oct United States Patent Olice 2,718,482 Patented Sept. 20, 1955 HEAT TREATMENT OF VANADIUM STEEL TO IMPROVE THE CREEP STRENGTH THEREOF Ernest A. Sticha, La Grange Park, lll., assignor to Crane Co., Chicago, lll., a corporation of Illinois Application October 31, 1950, Serial No. 193,234

1 Claim. (Cl. 14S-21.5)

This invention pertains to a heat treatment process applicable to steels, and, more particularly, it is concerned with a novel heat treatment of chromium-molybdenum vanadium-bearing steels.

It is an important object of this invention to provide a'heat treatment process of chromium-molybdenum vanadium-bearing steels whereby physical or mechanical properties of the steels are substantially improved, especially insofar as the desirable property of creep resistance is concerned.

Other objects and advantages will become more readily apparent upon proceeding with the following description read in light of the accompanying drawings, in which:

Fig. 1 is a graph showing physical properties of a chromium-molybdenum vanadium-bearing metal heat treated by normalizing and drawing as per the prior art.

Figs. 2 and 3 are graphs showing physical properties of the same steel used in Fig. 1, but heat treated in accordance with the concept of this invention.

Figs. 4, 5, and 6 are photo-micrographs of the same steel after being subjected to different heat treatments.

At the outset, a true appreciation of this inventive merit will be more readily obtainedv when considered in light of U. S. Patent #2,244,881 over which this invention is an The usual properties for the above steel when applied as high temperature bolting are as follows:

Tensile strength, min., pounds per sq. in 125,000 Yield point, min., pounds per sq. in 105,000 Elongation in 2 in., min., per cent 16 Reduction of area, min., per cent 50 Briefly, the heat treatment taught by the above-mentioned patent consists of normalizing from 1675 F. followed by drawing at 1200 F. Such heat treatment in comparison to oil quenching and drawing, produces desirable improvements in some of the physical or mechanical properties of the steel referred to, but other properties are adversely affected. For example, creep resistance of the steel is improved while ductility and toughness are hampered. The present invention produces the heat treated steel with increased creep resistance while giving a greater ductility and toughness than produced by the above mentioned treatment.

The essence of this invention embodies a heat treatment of the above steel wherein the lirst step is grain coarsening or austenitizing in the range of 1800 F. to 2200 F., to be followed, after air or furnace cooling to should be noted that all test data submitted herein isV based on lthe use of a 3%; inch diameter bar stock. Thus, the time of the holding period will change according to the mass or thickness of the steel, but, in using the 3A inch stock, a three hour period is adequate.

It has been discovered that the preferred austenitizing temperature for achieving the most desirable creep resistance is a temperature in the range of 2000 F. Test results which led to this discovery are hereinafter discussed.

-As previously mentioned, the higher heat treating or normalizing temperatures, such as those above 1675 F., when applied according to the prior teachings Vto chromium-molybdenum vanadium-bearing steels, a decided loss in ductility and toughness results. This loss is due to embrittlement of the steel, and it has been determined that the embrittlement is caused by solution of a vanadium compound, probably carbide, at higher temperatures and subsequent precipitation during the drawing operation. In considering the present invention of heat treating chromium-molybdenum vanadium-bearing steels, the grain coarsening procedure required in metals of high creep resistance, is desirably achieved by austenitizing well above the 1675 F. temperature to a range of 1800 F. to 2200 F. Upon cooling the metal to 1675 F., the approximately three hour holding period then permits the vanadium compound to separate out, thus avoiding the vanadium solution and consequent embrittlement.

In common with most metallic materials, creep strength may be improved through grain coarsening by the application of higher heat treating temperatures. But, as previously mentioned, the chromium-molybdenum vanadiumbearing steels subjected to higher normalizing temperatures suffer a loss of ductility and toughness. Fig. 1 graphically displays test data on the result of subjecting the chromium-molybdenum vanadium-bearing steel to increased austenitizing and normalizing temperatures. The loss in impact resistance should be especially noted.

For purposes of ready comparison, graphic data similar to that plotted in Fig. 1 is shown in Fig. 2 on the properties of the same steel subjected to the heat treatment of this invention. It should be noted that the same scales are used in both gures to further facilitate the comparison of important resultant properties after the different heat treatments.

As evidenced by the respective curves of the hardness number, the heat treated steel of Fig. 1 shows a definite reduction in ductility or toughness with increased normalizing temperatures whereas the steel of Fig. -2 shows only a negligible decrease in ductility. The maximum hardness number in Fig. 2 is approximately 312 VPN/ 30d, while the maximum hardness number in Fig. 1 is approximately 352 VPN/30d. Both of these maximum points occur at the higher temperatures of each treatment.

An analysis of the respective impact resistance curves of Figs. 1 and 2 shows a drastic reduction in resistance in Fig. 1, while Fig. 2 shows only a slight reduction in resistance with the increased austenitizing temperatures.

Still a further important mechanical property which should be noted and compared between the two heat treated steels is the tensile strength. As apparent in Fig. 2, the tensile strength of a steel subjected to the heat treatment of this invention is shown to increase but slightly while that of the heat treated steel in Fig. 1 increases only slightly more. Thus, the tensile strength of the steel exposed to the heat treatment taught herein is not appreciably changed from that produced by the low tcmperature normalizing of 1675 F. as now done by the prior art (see Fig. 1). Comparatively, the tensile strength of the steel treated by the prior art process shows an increase in strength, but, since losses in other properties have heretofore prohibited austenitizing above 1675 F., the tensile strength shown at that temperature in Fig. 1 is the maximum strength available. Thus, it is apparent that the available strength property of the steel may also be improved with heat treating according to this invention, since the maximum strength of Fig. 2 is well above that of the reading of 1675 F. shown in Fig. 1.

Thus, it is evident that the properties of desirable ductility and impact resistance of chromium-molybdenum vanadium steels may be satisfactorily retained through a heat treatment process. The following discussion will show the accompanying desirable improvement in creep resistance resulting from the heat treatment referred to above which is the one taught by this invention.

Increased creep resistance of the grain coarsened steel material is shown in Fig. 3 on a logarithmic scale. It should be noted that the creep strength of the steel austenitized at 2000" F., at a rate of 0.1% per 104 hours is approximately 30% greater than that of the steel normalized at 1675 F. as per the prior art. Also, at a given stress, corresponding strain is shown to be considerably greater for a steel austenitized andnormalized at 1675 F. over that of a steel austenitized at 2000 F. and normalized at 1675 F. Heat treatment at 1800 F. produces intermediate creep resistance values which check closely with results for steels normalized from the same temperature. Thus, analysis of the carefully acquired test results shown in Fig. 3 leads to the conclusion of improved creep strength of chromium-molybdenum vanadium-bearing steels heat treated under the teachings of this invention.

In addition to all of the foregoing benefits, it is discovered that much better ductility under elevated temperature conditions is realized by application of this special heat treatment. Creep rupture tests have been conducted at 1000 F. with heat treatments of both the prior art and this invention applied to steel. It has been discovered that steels austenitized and normalized from above 1675 F. show low percentages of elongation and relatively few hours to rupture under a given stress. Thus, proof of poor ductility is obtained. However, when the steel is subjected to an austenitizing temperature of approximate- Iy 2000 F. and normalized at 1675 F., there results higher percentage of elongation than the above and a greater number of hours are required prior to rupture. The conclusion is that greater ductility or less embrittlement results in a chromium-molybdenum vanadium bearing steel subjected to the heat treament of this invention.

As shown in Figs. 4, 5, and 6, photo-mcrographs at a magnification of 500 times display the various grain structures of metals subjected to diiferent heat treatments.

Fig. 4 is a photo-micrograph of the chromium-molybdenum vanadium bearing steel austenitized and normalized at 1675 F. with the result of a relatively fine grain structure which is associated with the corresponding creep resistance shown in Fig. 3. Fig. 5 shows the coarser grain structure of the steel when austenitizedand normalized at 2000 F. Although this steel possesses improved creep resistance, as previously pointed out, an accompanying loss of ductility and toughness will result. Fig. 6 shows the steel treated in accordance with this invention when austenitized at 2000 F. and normalized from 1675 F. It should be noted that the desirable coarse grain structure is acquired and, as previously seen, ductility and toughness are not impaired.

It should, of course, be understood that variations exist in the heat treatment as detailed above, and, thus this invention should be limited only by the scope of the appended claim and the spirit of this invention.

I claim:

A method of heat treating a chromium-molybdenumvanadium-bearing steel, having chromium within the range of .80% to 1.10%, molybdenum .30% to .40%, vanadium .20% to .30%, carbon about .35% to .50%, manganese about .40% to .70%, silicon about .15% to .30%, the remainder of the steel being substantially iron,

' the method including by heating for grain coarsening to a range between 1800 degrees F. and 2200 degrees F. and holding at a rate of approximately one hour per inch of diameter or thickness, cooling to about 1675 degrees F. and holding for a time sufficient to permit separation of the vanadium carbide, then normalizing from latter temperature and finally drawing said steel at about 1200 degrees F.

Kanter June 10, 1941 Payson Oct. 23, 1951 A", ...HM-c.

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