High strength, low alloy, heat resistant steel

Details High strength, low alloy, heat resistant steel High strength, low alloy, heat resistant steel

2000-08-18

2002-04-30

Yee, Deborah (Department: 1742)

Alloys or metallic compositions

Ferrous

Chromium containing, but less than 9 percent

C420S108000, C420S109000, C420S110000, C148S334000, C148S335000

Reexamination Certificate

active

06379611

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a high strength, low alloy, heat resistant steel which has excellent resistance to heat treatment cracks, i.e., it has a low susceptibility to cracks formed in welded joints during heat treatment after welding.
2. Description of the Related Art
Examples of well-known high temperature materials for use in heat resistant and pressure resistant piping for boilers, chemical equipment, and similar devices include low alloy, heat resistant steels containing a few % of Cr, high Cr ferritic heat resistant steels containing 9-12% of Cr, and austenitic stainless steels typified by 18% Cr-8% Ni steel. Of these materials, low alloy, heat resistant steel typified by 2.25% Cr-1% Mo steel (a so-called 2¼% Cr-1% Mo steel) is inexpensive, so it is used in large quantities.
Low alloy, heat resistant steels containing a few % of Cr typically have a ferrite texture such as tempered bainite or tempered martensite. Compared to high Cr heat resistant steels and steels with an austenitic texture, their high temperature strength is low. For this reason, in recent years, there have been numerous proposals for improving the high temperature strength of low alloy, heat resistant steels by adding Mo, W, Nb, V, and other alloying element to a low alloy steel.
For example, Japanese Patent No. 2659813 discloses a steel which contains Cr: 0.7-3%, Mo: 0.3-1.5%, V: 0.05-0.35%, Nb: 0.01-0.12%, and N: 0.01-0.05%, and which is further adjusted to contain W: 0.5-2.4%, B: 0.0005-0.015%, Al: at most 0.1%, and Ti: 0.05-0.2%. The steel is heated to a temperature of at least 1100° C. and is then cooled to room temperature, plastic working is carried out at room temperature or during working or during cooling in a temperature range in which recrystallization will not take place, and finally normalizing at a temperature lower than 1100° C. and tempering at not higher than the A
C1
temperature are carried out to manufacture a steel in which the degree of softening of weld heat affected zones is lessened and in which the impact properties of the base material are improved and which can be used at a temperature up to approximately 600° C.
Japanese Published Unexamined Patent Application Kokai Hei 4-268040 discloses a low alloy, heat resistant steel having excellent creep strength and toughness and which contains Cr: 1.5-3.5%, W: 1-3%, V: 0.1-0.35%, Nb: 0.01-0.1%, B: 0.0001-0.02%, N: less than 0.005%, Al: less than 0.005%, and Ti: 0.001-0.1%, and if necessary further contains one or more of La, Ce, Y, Ca, Zr, and Ta each in an amount of 0.01-0.2% and Mg in an amount of 0.0005-0.05%, and further contains Mo: 0.01-0.4%, with the amounts of Ti and N satisfying the formula:
0.080≧Ti(%)−(48/14)N(%)≧0.003.
Japanese Published Unexamined Patent Application Kokai Hei 5-345949 discloses a low Cr ferritic heat resistant steel having excellent toughness and creep strength which includes Cr: 1.5-3.5%, W: 1.0-3.0%, V: 0.10-0.35%, Nb: 0.01-0.10%, B: 0.0001-0.02%, N: less than 0.005%, Al: less than 0.005%, Ti: at least 0.001% and less than 0.050%, Cu: 0.10-2.50%, and if necessary contains Mo: 0.01-0.40% and one or more of La, Ce, Y, Ca, Zr, and Ta each in an amount of 0.01-0.20% and Mg in an amount of 0.0005-0.05%, and among impurities, P: at most 0.03%, and S: at most 0.015%.
This steel has a high creep strength, so the N content and the Al content are limited to less than 0.005%, and Ti is added to fix N, and B is added. In addition, in order to improve the toughness of welds, Ti, Cu, and W are added. Cu is added in order to improve oxidation resistance and corrosion resistance, while V, Nb, and W are added to improve strength.
Japanese Published Unexamined Patent Application Kokai Hei 8-325669 discloses an ultra-low Mn, low Cr ferritic heat resistant steel having excellent high temperature strength which contains Cr: 0.8-3.5%, W: 0.01-3.0%, V: 0.1-0.5%, Nb: 0.01-0.20%, Al: 0.001-0.05%, Mg: 0.0005-0.05%, B: 0.0020-0.02%, N: less than 0.005%, P: at most 0.03%, and S: at most 0.015%, and if necessary contains Mo: 0.01-1.5%, and one or more of La, Ce, Y, Ca, and Ta each in an amount of 0.01-0.2%, and a remainder of Fe and unavoidable impurities, wherein the B content satisfies the formula:
(14/11)B>N−N(V/51)/{(C/12)+(N/14)}−N(Nb/93)/{(C/12)+(N/14)}.
In this steel, W is added to improve high temperature creep strength, the amount of Mn is limited to less than 0.1% in order to suppress a decrease in the effect of W after long periods of use, and B is added to prevent a decrease in strength and toughness and to increase high temperature creep strength. In order to guarantee the effectiveness of B, the amount of B is controlled by the above formula relating N, V, C, and Nb.
Japanese Published Unexamined Patent Application Kokai Hei 10-8194 discloses a ferritic steel having excellent weldability and high temperature strength which includes Cr: 0.3-1.5%, W: 0.1-3%, V: 0.01-0.5%, Nb: 0.01-0.2%, Al: 0.001-0.05%, B: 0.0001-0.02%, N: 0.001-0.03%, P: at most 0.025%, and S: at most 0.015%, if necessary one or more of Mo: 0.01-3%, Ca, Ti, Zr, Y, La, Ce, and Ta each in an amount of 0.01-0.2% and Mg in an amount of 0.0005-0.05%.
This steel is a low Cr ferritic steel which can be used in place of a high Cr ferritic steel and which has improved high temperature creep strength at a temperature of at least 450° C. and which has performance comparable to or better than that of existing low alloy steels with respect to toughness, workability, and weldability.
When performing welding with low alloy, heat resistant steels like those described above, there is the problem that weld metal cold cracks develop. In order to prevent weld metal cold cracks, it is typical to perform preheating before welding and then to perform heat treatment after welding. However, as described on pages 10, 22-23, 100, and 150 of “Standards for Heat Treatment after Welding and their Explanation” (Japan High Pressure Technology Organization, edited by the Stress and Annealing Working Group, published by Nikkan Industrial Newspaper on Sep. 26, 1994), it is known that cracks develop in the weld heat affected zone of these low alloy, heat resistant steels during heat treatment after welding, i.e., they have a high susceptibility to heat treatment cracks. Heat treatment cracks are produced by a different mechanism from weld metal cold cracks, so they cannot be prevented by control of the preheating temperature.
Many reports have been published concerning heat treatment cracks in low alloy, heat resistant steels. For example, a heat treatment crack susceptibility index (P
SR
) is proposed in Journal of Welding Academy, Volume 41 (1972), No. 1, page 59. For a Cr content in the range of at most 1.5%, the heat treatment crack susceptibility index increases with increases in the amounts of Cr, Cu, Mo, V, Nb, and Ti. In particular, V, Nb, and Ti have a large effect on the index. In addition, Journal of Welding Academy, Volume 49, (1980), No. 3, page 203 discloses that the heat treatment crack susceptibility index increases as the amounts of the impurities P, Sb, Sn, and As increase in steel. Furthermore, Japanese Published Unexamined Patent Application Kokai Sho 59-80755 proposes a low alloy, heat resistant steel having excellent resistance to temper brittleness.
SUMMARY OF THE INVENTION
Each of the above-described publications concerning heat treatment cracks relates to a steel which does not contain W. As a result of studies by the present inventors, it became clear that in the case of a steel containing W, the strength at high temperatures is high, so the susceptibility to heat treatment cracking is markedly increased.
In the publications listed above, except for Japanese Published Unexamined Patent Applications Kokai Hei 4-268040 and Kokai Hei 5-345949, there is no description concerning welding cracks. In Japanese Published Unexamined Patent Applications Kokai Hei 4-268040 and Kokai Hei 5-345949, there is a description concerning prevention of weld met

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