Method of heat treating a martensitic chromium-molybdenum steel

Abstract

 Method of heat treating a martensitic chromium-­molybdenum steel by austenitizing the steel at a tempera­ture of from 1100 to 1200°C, cooling to a temperature less than 100°C, and tempering the steel at a temperature of from 650 to 705°C. The steel has a composition of from 0.17% to 0.23% carbon, from 0.4% to 0.7% manganese, up to 0.03% phosphorus, up to 0.02% sulfur, from 0.2 to 0.5% silicon, from 0.3 to 0.8% nickel, from 11 to 12.5% chromi­um, from 0.8 to 1.2% molybdenum, up to 0.05% cobalt, from 0.4 to 0.6% tungsten, up to 0.05 aluminum, from 0.25 to 0.35% vanadium, and the balance iron.

Description

[0001] This invention relates to a method of heat treating a martensitic chromium-molybdenum steel.

[0002] Fuel rod cladding for use in a nuclear reactor consists of very long narrow tubes in which is placed pellets of fissionable material. Metals used for fuel rod cladding must be able to withstand extreme conditions of temperature, pressure, and radiation, while in contact with corrosive or chemically active materials. One material which has been found to meet these difficult requirements is an iron-nickel-chromium alloy described in U.S. Patent Specification No. 4,578,130. While that material has excellent properties for use as fuel cladding, it has nevertheless become desirable to increase the stress rupture strength of the tubes to reduce the likelihood that the tubes will rupture, permitting the escape of fission gases, which might necessitate shutting down the reactor. While this could be accomplished by increasing the thick­ness of the tubes or by changing the composition of the alloy, any change such as this would create other adverse effects.

[0003] Accordingly, the present invention resides in a method of heat treating a martensitic chromium-molybdenum steel characterized by austenitizing said steel at a temperature of from 1100 to 1200°C, to an ASTM grain size of from 7 to 9; cooling said steel to a temperature less than 100°C; and tempering said steel at a temperature of from 650 to 705°C.

[0004] The stress rupture time of a stainless steel alloy as described in U.S. Patent Specification No. 3,917,492 can be increased by a factor of from 5 to 8 by the relatively simple procedure of the last preceding paragraph. This procedure makes it possible to increase the stress rupture strength without increasing the thick­ness of the cladding, without changing the composition of the alloy, and without adversely affecting the other properties of the cladding.

[0005] The accompanying drawing is an isometric view in section showing a certain presently preferred embodiment of a fuel rod cladding tube according to this invention. In the drawing, a fuel rod cladding 1 consists of a thin narrow tube which is filled with pellets 2 of a fissionable material.

[0006] The method of this invention is applicable to any martensitic chromium-molybdenum steel. A typical composi­tion for such a steel is from 0.17% to 0.23% carbon (all percentages are by weight based on total composition weight), from 0.4% to 0.7% manganese, up to 0.03% phospho­rus, up to 0.02% sulfur, from 0.2 to 0.5% silicon, from 0.3 to 0.8% nickel, from 11 to 12.5% chromium, from 0.8 to 1.2% molybdenum, up to 0.05% cobalt, from 0.4 to 0.6% tungsten, up to 0.05 aluminum, from 0.25 to 0.35% vanadium, and the balance iron.

[0007] The steel is made into tubes suitable for use as fuel cladding, and then the tubes are heat treated accord­ing to this invention. The tubes for use as fuel cladding are typically from 0.2 to 0.6 inches in outside diameter, from 3 to 12 feet long, and from 0.01 to 0.03 inches thick. The accompanying drawing is an isometric view, in section, of a fuel rod cladding 1 consisting of such a thin narrow tube which is filled with pellets 2 of a fissionable material.

[0008] In the heat treatment according to this inven­tion, the steel is placed in an oven and is austenitized (which can also be called "solution annealing" or "normal­izing") at a temperature of from 1100 to 1200°C. It is preferable to treat the alloy in tube form, although the benefits of this invention can be achieved when the alloy is in sheet or other forms. The austenitizing proceeds until the steel has achieved a grain size of from 7 to 9 according to ASTM Standard E112, which typically requires from 2 to 10 minutes. A preferred austenitizing tempera­ture, which tends to maximize the stress rupture strength/­life, is from 1100 to 1150°C. The austenitization of the steel, i.e., the conversion of the steel to an austenitic structure, is, of course, critical to obtaining the drama­tic increase in stress rupture strength/life achieved by the method of this invention. The austenitization is preferably performed by placing the steel tube inside of a quartz tube which is heated to the austenitizing tempera­ture. Titanium, tantalum, or zirconium can be added as an oxygen getter and the quartz tube is backfilled with an inert gas such as argon. It is also possible to use a vacuum furnace for austenitizing or to use a hydrogen atmosphere in the furnace to produce bright hydrogen annealing. In any case, it is essential to keep oxygen out during the austenitizing process to avoid oxidation and decarburization.

[0009] After it is austenitized, the steel is cooled to a temperature below 100°C, and preferably to room tempera­ture. This cooling step is critical because it is required to ensure that a martensite transformation has gone to completion. The martensite transformation creates the high strength of the alloy. At the completion of the austenitization, the steel is tempered at a temperature of from 650 to 750°C, and preferably at from 670 to 680°C. The tempering temperature should be regarded as highly critical to obtaining a large improvement in stress rupture strength/life as these temperature ranges tend to result in the maximum increase in stress rupture strength. The tempering typically requires from 30 to 120 minutes; longer times can be used but are usually unnecessary. The process can be most easily performed by moving the steel on a movable belt through the austenitizing furnace, into room temperature air for cooling, followed by moving into the tempering furnace.

[0010] The invention will now be illustrated with reference to the following Example:

EXAMPLE

[0011] Standard biaxial stress rupture tests were conducted on 0.230 inch × 0.200 inch, and 0.270 inch × 0.226 inch, cladding according to the procedures given in an Article by N. F. Panayotou and D. R. Duncan, titled "Biaxial Stress Rupture Behavior of the Clinch River Breeder Reactor Radial Blanket Rock Cladding", (TC-1304) December 1978, herein incorporated by reference. The cladding was a martensitic chromium-molybdenum steel alloyed with tungsten and vanadium, having a nominal chemical composition of 0.20% carbon, 0.4% silicon, 0.6% manganese, 0.030% maximum phosphorus, 0.020% maximum sulfur, 11.5% chromium, 0.5% nickel, 1.0% molybdenum, 0.5% tungsten, 0.3% vanadium, and 0.020% maximum cobalt, sold by the Sandvik Company as "SANDVIK HT9." The heat treatments applied to the cladding prior to testing are shown in the following table:

[0012] In the Table 1, "h" is hours, "m" is minutes, "AC" is air cooled, and "TMT" is thermomechanical treatment. The following Table gives the pretest grain sizes and average hardness measurements for each cladding condition:

[0013] The data in Table 2 were obtained on cladding in the standard condition which had been re-heat treated as indicated. In the above table, the first number is the hardness and the second is the grain size.

[0014] It is evident from Table 2 that some grain growth occurred as the normalization temperature increased, although an ASTM grain size of 5 was still judged to be acceptable for cladding.

[0015] To quantify the normalization treatment response, cladding specimens identified by the vendor as "heat number 84425" in the as-drawn condition (20% cold worked) were heat treated at temperatures ranging from 950 to 1200°C and the resulting microstructures were examined using optical and transmission electron microscopy.

[0016] A total of forty-nine valid tests were completed on re-heat treated cladding, generating data out to 2000 hours at 650 and 704°C. An additional sixteen tests were performed on the 0.270 inch and 0.226 inch cladding which was received in the 20% cold worked condition and which was given a particular subset of heat treatments prior to testing. The latter specimens are referred to as "cold worked and heat treated". The results of these experiments were given in the following tables, where 1, 2, 3, or 4 specimens were tested at each temperature:

[0017] In Tables 3 and 4, "MPa" is mega pascals.

[0018] Table 5 presents a summary of the performance of HT9 with standard TMT and the improved TMT. The last column shows that this new TMT will increase rupture life by a factor of 4 to 5.

In Table 5, "t_r" is stress rupture time.

Claims

1. A method of heat treating a martensitic chromium-molybdenum steel characterized by austenitizing said steel at a temperature of from 1100 to 1200°C, to an ASTM grain size of from 7 to 9; cooling said steel to a temperature less than 100°C; and tempering said steel at a temperature of from 650 to 705°C.

2. A method according to claim 1, characterized in that the tempering is performed for from 30 to 120 minutes.

3. A method according to claim 1 or 2, charac­terized in that the austenitizing is performed in hydrogen.

4. A method according to claim 1, 2 or 3, characterized in that the steel consists essentially of from 0.17% to 0.23% carbon, from 0.4 to 0.7% manganese, up to about 0.03% phosphorus, up to 0.02% sulfur, from 0.2 to 0.5% silicon, from 0.3 to 0.8% nickel, 11 to 12.5% chromi­um, from 0.8 to 1.2% molybdenum, up to 0.05% cobalt, from 0.4 to 0.6% tungsten, up to 0.05% aluminum, from 0.25 to 0.35% vanadium, and the balance iron.

5. A method according to any of claims 1 to 4, characterized in that the steel is in the form of tubes of from 0.2 to 0.6 inches in outside diameter, from 3 to 12 feet long, and from 0.01 to 0.03 inches thick.

6. A method according to any of claims 1 to 5, characterized in that the austenitizing is performed at from 1100 to 1120°C, and the tempering is performed at from 670 to 680°C.

Drawing