GB1574965A - Heat resisting low carbon alloy steels - Google Patents

Abstract

The steel alloy contains as essential components: a) from 0.10 to 0.20% by weight of carbon from 0.30 to 0.80% by weight of manganese from 1.00 to 1.50% by weight of chromium from 0.45 to 0.65% by weight of molybdenum from 0.40 to 0.75% by weight of silicon from 0.01 to 0.05% by weight of titanium b) phosphorus and/or antimony and/or tin in a total amount of from 0.01 to 0.05% by weight and c) the remainder being, at least for the greater part, iron. The alloy has high creep strength and breaking strength and a high fracture ductility at 538 DEG C. A heat resistant alloy having this composition and these properties is prepared by an alloy which 1) has a high impurity content of from 0.01 to 0.05% by weight of phosphorus and/or antimony and/or tin and 2) from 0.10 to 0.20% by weight of carbon from 0.30 to 0.80% by weight of manganese from 1.00 to 1.50% by weight of chromium from 0.45 to 0.65% by weight of molybdenum from 0.40 to 0.75% by weight of silicon 3) the remainder being iron, at least for the greater part, being admixed, prior to casting, with from 0.01 to 0.05% by weight of titanium and optionally from 0.004 to 0.008% by weight of boron.

Description

(54) HEAT RESISTING LOW CARBON ALLOY STEELS (71) We, WESTINGHOUSE ELECTRIC CORPORATION of Westinghouse Building, Gateway Center, Pittsburgh, Pennsylvania, United States of America, a company organised and existing under the laws of the Commonwealth of Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to heat resisting, low carbon alloy steels. Commercial low carbon alloy steel of that type generally designated 1.25 Cr, 0.5 Mo is referred to on page 467 of the ASM "Metals Handbook", 8th Edition, Volume 1, 1961. The handbook gives the nominal composition for the alloy as: 0.15% max. C. 0.45% Mn; 0.75% Si; 1.25% Cr; 0.5% Mo;and balance Fe. The 1.25 Cr 0.5 Mo alloy steel had been widely used in recent years as a material for pressure vessels, piping, tubing and steam turbine casings. While the commercial grade alloy exhibits good high temperature properties, a potential limitation to the use of this alloy at temperature above approximately 1 .0000F (5380C) is the problem of creep. Heat treatment procedures that permit realization of higher strength levels, unfortunately, lead to a loss in ductility and render the steel notch sensitive. Improvements in creep and rupture strength would not only enable use of this alloy at higher temperatures but also would reduce section thickness requirements, thereby leading tos:avings in weight and in material.

.Commercially used 1.25 Cr - 0.5 Mo alloy steels contain several embrittling elements such as phosphorus, antimony and tin, which at high temperatures tend to segregate at grain boundaries to cause embrittlement and subsequent weakening of the boundaries. The high temperature properties of the alloy could be improved if these impurities were removed, however, it is not economically feasible in commercial steel melting practice to remove the embrittling impurity elements to the very low levels required.

Heretofore it has been proposed to add boron, titanium, vanadium, copper, or columbium to alloys of this general type to improve the low temperature impact strength and weldability thereof, for example, see: U.S.

Patent No. 3,251,682 and U.S. Patent No.

3,288,600. The present invention solves the problem of high temperature embrittlement in the 1.25 Cr - 0.5 Mo alloy by providing controlled additions of titanium or titanium and boron to improve the high temperature properties as well as the room temperature properties of the alloy even in the presence of embrittling impurities phosphorus, antimony and tin in the steel.

Accordingly the present invention resides in a low carbon alloy steel characterized by high creep and rupture strengths and high rupture ductility at 10000F (538"C), said alloy consisting of from 0.10% - 0.20% by weight carbon; from 0.30% - 0.80go by weight manganese from 1.00% - 1.50% by weight chromium; from 0.45% - 0.65% by weight molybdenum; from 0.40% - 0.75% by weight silicon; from 0.01% - 0.05% by weight titanium; at least one of phosphorus, antimony and tin in an aggregate arnount of from 0.01% - 0.05% by weight, the balance being iron.

The invention includes a method of making a low alloy heat-resisting steel characterized by high creep and rupture strengths and high rupture ductility at 10000F (538"C) and characterized by having a high impurity level composition of from 0.01% and 0.05% of at least one of phosphorus, antimony and tin, the steel being further characterized by having a composition of from 0.10% -0.20% by weight carbon; from 0.30% -0.80% by weight manganese; from 1.00% - .OOSo 1.50% by weight chromium: from 0A5% - 0.65% by weight molybdenum; from 0.40% - 0.75% by weight silicon; and the balance being iron; which comprises adding to the alloy prior to casting from 0.01% to 0.05% by weight of titanium.

Desirably, the alloys contains 0.004 to 0.008% by weight of boron.

In a preferred embodiment of the invention carbon, manganese, chromium, molybdenum, silicon, titanium and boron are present in an amount of approximately 0.15, 0.60, 1.25, 0.50, 0.45, 0.03 and from 0.004 to 0.006% by weight respectively. The invention provides an alloy which exhibits higher hardness and tensile strength at room temperature with no decrease in percent elongation and percent reduction in area. The addition of titanium and boron provides an alloy exhibiting a significant improvement in rupture strength at 10000F.

The alloy of this invention containing titanium or titanium and boron exhibits high levels of rupture ductility in stress rupture tests run at 10000F for time periods up to 1,000 hours.

The beneficial effects of titanium or titanium and boron additions occur independent of the amounts of embrittling impurities; phosphorus, antimony and tin, present in the alloy.

The invention will now be illustrated with reference to the following Example: EXAMPLE In an effort to study the high temperature properties, and particularly the creep rupture ductility of commercial grade 1.25 Cr - 0.5 Mo low alloy steels, 8 heats were produced by vacuum induction melting. The chemical analyses of these heats are reported below in Table 1. Heat VM 1706 represented a high purity, control alloy containing low amounts of the embrittling elements phosphorus, antimony and tin. VM 1711 and 1712 contained titanium or titanium and boron, respectively, with antimony and tin as.principal impurities.

VM 1707 contained boron alone, with tin being the principal impurity. VM 1713 and 1714 provide a comparison between a titanium addition and a titanium plus boron addition with phosphorus as the principalimpurity. VM 1715 contained no titanium or boron and contained high levels of phosphorus, antimony and tin.

VM 1716, likewise, contained high levels of the embrittling impurities plus an addition of boron.

The vacuum induction melted heats were cast into 2 inch by 2 inch ingots and thereafter forged to 5/8 inch square bar stock at 2000"F (1093"C). Specimen blanks from the bar stock were normalized at 17000F (9250C) for one hour and tempered at 1 2500F (6750C) for one hour and water quenched. A commercial 1.25 Cr - 0.5 Mo alloy steel was also tested and was in the form of a 2 inch square keel block casting.

The commercial alloy was normalized at 1 7000F (9250C) and tempered subsequently at 13000F (7040C). The lower tempering temperature for the laboratory steels was chosen mainly to produce higher strength levels, thereby accelerating any possible embrittlement effect in short time tests. Test specimens were machined from the specimen blanks following the final heat treatment. Specimens had a gauge and notch diameter of 0.357 inches (0.907 cm).

The notch had a root radius of 0.010 inches (0.0254 cm) and a major diameter of 0.5 inches (1.27 cm). This geometry produced a theoretical stress concentration factor of 4.

Creep tests were run in air in lever arm type machines at 10000F (5380C). Tensile test data were obtained at room temperature.

Results from tensile and hardness tests at room temperature for the various alloys are listed below in Table 2. Among the experimental heats, VM 1712, 1713 and 1714 exhibited considerably enhanced room temperature strength, without any appreciable reduction in ductility thus indicating the beneficial effects due to additions of titanium or titanium and boron. Heat VM 1711 is an exception to this and exhibited slightly lower room temperature strengths relative to the control steel VM 1 706.

The tensile strength levels at room temperatures achieved by VM 1712, 1713 and 1714 represents an improvement of at least 1 5% relative to the control heat VM 1706 and about 35% relative to the commercial alloy steel. All tensile speci mens failed in the smooth bar section indicating absence of any notch sensitivity at room temperature.

As can be noted from the above Tables heat VM 1714 was the strongest alloy at room temperature. VM 1714 contained 0.16% carbon; 0.57% manganese; 1.24% chromium; 0.5% molybdenum; 0.45% silicon; 300 ppm or .03% titanium; 55 ppm or .0055% boron. The major impurity was phosphorus in an amount of 320 parts per million or .032%, the balance was essentially iron with trace impurities of anti mony, tin, sulfur, nitrogen and oxygen. Phos phorus and tin appear to have an effect on the strength level at room temperature. VM 1714, containing both titanium and boron with a high phosphorous and low tin content, achieved higher strength levels than heat VM 1712 which contained similar amounts of titanium and boron with low phosphorous and high tin. This apparent relationship is further demonstrated by a comparison of VM 1713 and VM 1711 which contained titanium and no boron.

VM 1713 with high phosphorous and low tin and low antimony was substantially stronger than VM 1711 which contained low phosphorus moderate amounts of antimony and high tin.

The combined amount of antimony and tin in VM 1713 was 0.0024% by weight and we have found that this combined amount may be in TABLE 1 Chemical Composition of Experimental Steels % PPM Steel C Mn Cr Mo Si Ti B P Sb Sn S N O VM 1706 (control) 0.15 0.58 1.25 0.49 0.45 -- -- 20 4 20 30 5 25 VM 1707 0.16 0.58 1.22 0.49 0.45 -- 45 20 5 320 32 20 23 VM 1711 0.16 0.59 1.17 0.49 0.45 300 -- 20 100 330 25 10 15 VM 1712 0.16 0.56 1.24 0.49 0.45 300 60 10 93 280 27 6 23 VM 1713 0.16 0.55 1.24 0.49 0.45 300 -- 300 3 21 26 14 37 VM 1714 0.16 0.57 1.24 0.50 0.45 300 55 320 2 23 26 8 25 VM 1715 0.17 0.55 1.25 0.50 0.45 -- -- 270 94 350 22 6 50 VM 1716 0.16 0.56 1.25 0.49 0.45 -- 40 300 97 325 26 7 33 TABLE 2 Results of Hardness and Tensile Tests at Room Temperature on Experimental Steels Ultimate Tensile 0.2% Yield Elongation, Reduction Steel Hardness, RB Strength, ksi Strength, ksi % in Area, % VM 1706 94.5 92.3 74.4 26.9 73.9 VM 1707 94.0 92.3 74.4 25.7 70.9 VM 1711 91.5 88.0 69.4 19.0 71.6 VM 1712 97.0 102.7 88.9 24.9 64.8 VM 1713 97.0 105.7 90.6 22.0 68.4 VM 1714 98.6 108.4 93.7 20.4 65.5 VM 1715 94.6 92.7 71.8 25.3 69.8 VM 1716 92.5 94.0 85.8 26.3 69.2 Commercial -- 77.5 54.0 29.0 71.9 1.25 Cr - 0.5 Mo creased up to 0.003% while still retaining the strength advantages.

The improved rupture strength characteristics at elevated temperatures for the alloys of this invention may be demonstrated by referring to Figure 1. Stress rupture tests were performed at 1000"F (5380C) on control alloy VM 1706, and the two alloys containing titanium and boron additions, VM 1712 and VM 1714. The tests were run utilizing four levels of stress, namely, 35 ksi, 38 ksi, 44 ksi and 54 ksi. Alloy VM 1712 having the high tin, antimony content exhibited the best stress rupture characteristics in this test. VM 1714 containing high phosphorus also exhibited good stress rupture characteristics, above that of the high purity control alloy VM 1706. It is seen from Figure 1, that the addition of titanium and boron results in a significant improvement in rupture strength even in the presence of embrittling impurities. It is estimated that the 10,000 hour rupture strength of heats VM 1712 and 1714 are at least 20% higher compared to the control steel VM 1706.

Figure 2 illustrates observed variation of percent reduction in area at rupture for the alloys as a function of log time to rupture in hours at 10000F (5380C). These tests demonstrate improved rupture ductility as a result of titanium and titanium plus boron additions. The plot for the control steel VM 1706 is characterized by an initial region of constant ductility followed by a region of decreasing ductility at tr in excess of about 100 hours. For the alloys of this invention, the ductility values are in the narrow range of from 80% to 90% and show no tendency to decrease for times in excess of 1000 hours. For example, VM 1712 hada value of 81% reduction in area, even at 2300 hours. It is interesting to note that alloys VM 1707 and 1716, containing boron alone with high impurity levels, exhibited a very rapid decrease in ductility after 100 hours, more rapid than the control heat VM 1706.

VM 1715 was similar in composition to the control heat but contained higher levels of phosphorus antimony and tin which apparently caused a more rapid decrease in ductility after 100 hours.

It is theorized that the impurity elements segregate at the grain boundaries at high temperatures and cause a weakening of the grain boundaries thus decreasing the creep rupture ductility of the alloy. This effect is overcome by additions of titanium and boron which reduce the susceptibility to intergranular fracture even in the presence of these grain boundary embrittlers.

Variation of log minimum creep rate with stress is plotted in Figure 3 in which the difference in creep strength at 1 ,0000F between several heats can be observed. The minimum creep rates are consistently smaller by at least a factor of 3 for both VM 1712 and 1714 compared to the control alloy VM 1706 indicating significant improvements due to the titanium and boron additions. The effect of the titanium addition alone, in the absence of boron, is not quite understood and varies with the nature and amount of the other impurities present. For example, VM 1711 which contained titanium in presence of antimony and tin exhibited consistently lower creep strength while VM 1713 which contained titanium and high phosphorus possessed consistently improved creep strength relative to the control alloy VM 1706.

The 1.25 Cr - 0.5 Mo low carbon alloy steels are generally used under conditions of high temperature and high stress. One such application is for steam turbine casings. These casings are cast and, in use, they are subjected to the high pressures of supeiheated steam and high temperatures in the 10000F11000F range. In order to determine if the properties of the cast alloy would be comparable to the forged specimens, additional tests were run on cast specimens. The materials utilized in this evaluation were from half ingots saved from corresponding heats in the prior forging study.

The heat treatment and test procedures carried out on the cast specimens were identical to those used earlier on the forged specimens.

TABLE 3, below, reports the results of the creep tests at 10000F at 35 ksi on forged and cast specimens for heats VM 1706, 1713, 1712, and 1714 along with the commercial alloy.

The results reported in TABLE 3 indicate that in the forged as well as in the cast conditions, the steels containing titanium and boron, VM 1712 and 1714, are superior to both the control heat VM 1706 and the commercial alloy in terms of creep strength, ultimate tensile strength, rupture strength, and ductility.

Based on the above results, it is seen that appreciable improvements can be obtained in room temperature strength as well as in stress rupture strength at 10000F in 1.25 Cr - 0.5 Mo low carbon alloy steels by controlled additions of titanium or titanium and boron. The data indicate that increases in tensile strength and rupture strength appear to be related. Since creep strength is also related to stress rupture strength, the three parameters can be grouped together under the term "strength" for convenience in discussing the results. Test results of VM 1712 and 1714 show a marked improvement in strength of the steel alloy due to the titanium and boron addition, regardless of the amount of embrittling elements phosphorus. antimony and tin present. VM 1713 having titanium with high phosphorus also showed improved strength compared to VM 1706 (control). VM 1711 containing titanium with high impurity contents of antimony and tin, on the other hand, had lower room temperature strength than the control VM 1706.

The results indicate that the alloy of this invention provides improved rupture strength at 1000 F and improved room temperature tensile strength with no accompanying loss of ductility TABLE 3 Results of Creep Tests at 1000 F and 35 ksi Final Hardness Creep Rate tr % RA Steel Condition VHN %/hr hr at Rupture VM 1706 Forged 202 .0033 1111 63 (Control) Cast 198 .003 664 79 VM 1713 Forged 246 .0013 1156 83 (Ti) Cast 224 .0042 571 83 VM 1712 Forged 233 .00045 2306 81 (Ti + B) Cast 218 .0024 1030 83 VM 1714 Forged 251 .0011 1301 84 (Ti + B) Cast 237 .0009 1511 73 Commercial Cast -- -- 1000 88 by the combined additions of titanium and boron. Furthermore, additions of titanium or titanium and boron result in significant improvements in stress rupture ductility at 10000F despite the presence of large amounts of embrittling impurities such as antimony, phosphorus and tin in the steel.

Claims (8)

WHAT WE CLAIM IS: -

1. A low carbon alloy steel characterized by high creep and rupture strengths and high rupture ductility at 1000 F (538 C), said alloy consisting of from 0.10%-0.20% by weight carbon; from 0.30%-0.80% by weight manganese; from 1.00%-1.50% by weight chromium; from 0.45%-0.65% by weight molybdenum; from 0.40%-0.75% by weight silicon; from 0.01%-0.05% by weight titanium;at least one of phosphorus, antimony and tin in an aggregate amount of from 0.01%-0.05% by weight; the balance being iron.

2. An alloy steel according to claim 1, wherein the silicon content is approximately 0.45% by weight.

3. An alloy steel according to claim 1 or 2, wherein from 0.004% to 0.008% by weight boron is also present.

4. An alloy steel according to claim 3, wherein there is present approximately 0.15% by weight carbon; approximately 0.60% by weight manganese; approximately 1.25% by weight chromium; approximately 0.50So by weight molybdenum; approximately 0.03% by weight titanium; and from 0.004%-0.006% by weight boron.

5. An alloy steel according to any of claims 1 to 4 wherein there is present approximately 0.03% phosphorus and a combined antimony and tin content of less than 0.003% by weight.

6. A method of making a low alloy heatresisting steel characterized by high creep and rupture strengths and high rupture ductility at 10000F (5380C) and characterized by having ahigh impurity level, composition of from 0.01% to 0.05% in total of at least one of phosophorus, antimony and tin, the steel being further characterized by having a composition of from 0.10%-0.20% by weight carbon; from 0.30-0.80% by weight manganese; from 1.00%-1.50% by weight chromium; from 0.45%-0.65% by weight molybdenum; from 0.40%-0.75% by weight silicon; and the balance being iron; which comprises adding to the alloy prior to casting from 0.01% to 0.05% by weight of titanium.

7. A method according to claim 7, which comprises also adding to the alloy prior to casting from 0.004% to 0.008% by weight of boron.

8. Low carbon alloy steels as claimed in claim 1 and substantially as described herein with particular reference to the foregoing Example.