GB2204060A - Copper modified austenitic stainless steel alloys with improved high temperature creep resistance - Google Patents

Description

2204060 COPPER MODIFIED AUSTENITIC STAINLESS STEEL ALLOYS WITH IMPROVED

HIGH TEMPERATURE CREEP R=rARM

BACKGROUND OF THE INVENTION

This invention relates to austenitic stainless steel alloys which have improved high temperature creep resistance. The alloys comprising the invention are basically nickel-chromium steel alloys which have closely controlled additions of various minor alloying constituents, especially including the deliberate addition of copper. These minor alloying constituents, in the proper quantities, provide the resulting alloys with improved thermal creep resistance and were developed pursuant to a contract with the U.S. Department of Energy.

The invention is a response to a continuing need for improved steel alloys for use in high temperature environments. This need is apparent in the area of advanced steam cycle fossil energy plants since the high operation temperatures and steam pressures required for such facilities are damaging to existing steel alloys. These steels may also be applicable to the need for better radiation resistant materials for both nuclear fission and fusion reactors as well.

Currently research is underway to develop austenitic p1loys for use as superheater/reheater materials in advanced steam cycle fossil energy plants. Such plants require a new alloy capable of withstanding higher 2 steam temperatures and pressures (6SO'C, 35MPa) without disadvantages characteristic of materials now in use. Currently, 300-series austenitic stainless steels like types 304, 316, 321 and 347 can be used with 540% 24MPa steam. More exotic superalloys like INCONEL 617 (a product of the International Nickel Co.) could be used in advanced steam cycle plants, but from the standpoints of lower cost.and conservation of strategic materials such as Cr, Co, and Ni, it is desirable to use a "lean" stainless steel, that is, one in which concentrations of chromium and nickel are lower, in the 12-18 wt% and 8-16 wt% ranges, respectively. Some of the lean stainless steels developed ip the last 30 years included 17-14 CuMo, Sandvik 12R72HV (a product of Sandvik), and the Japanese Tempaloys-Al and -A2 (a product of Nippon-Kokan Steel Company). New advanced alloys that are developed must exceed the performance of these current alloys at similar cost in order to be competitive in an open market.

A previously developed advanced modified lean austenitic stainless steel with improved radiation resistance is disclosed in a patent applica tion filed on February 11, 1987, by the U.S. Department of Energy attorney Docket No. DOE S-63,612; it comprises a Fe-16Ni-14Cr base alloy composition with 2.5 Mo and 2 Mn (all in wt%), and controlled minor alloying additions of V, Ti, Nb, C, N, P, B and Si, but with the amount of silicon being kept lower than the maximum limit normally allowed in 300 series stainless steels. Although an improvement in thermal creep resistance over other steels, the previously developed modified stainless steel has not yet demonstrated the degree of high temperature creep resistance that the inventors have achieved with the invention of this application. Consequently, there remains a continuing need for improved 3 steel alloys which offer even greater high temperature creep resistance than existing alloys, apart from their ability to resist radiationinduced degradation due to swelling or helium embrittlement.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an austenitic stainless steel alloy which has improved resistance to thermal creep.

It is another object of the present invention to provide an austenitic stainless steel alloy which may have improved resistance to radiation-induced degradation due to swelling and embrittlement.

A further object of the present invention to provide an austenitic stainless steel alloy with improved durability and effective life in both radiation and high temperature environments.

It is still another object 9f the present invention to provide an austenitic stainless steel alloy with improved physical properties for use in nuclear engineering applications.

Another object of this invention is to provide an austenitic stain-' less steel with improved creep resistance at higher temperatures.

It is an additional object of the present invention to provide an austenitic stainless steel alloy which improved physical properties that can be produced economically with conventional technology.

Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the Invention my he realized and attained by means of instrumental ities and combinations pa.rticularly pointed out in the appended cl aims.

These objects are achieved by providing an austenitic stainless steel alloy consisting essential.ly of Iron, nickel, and chronium, with the addition of closely controlled minor alloying quantities of molybdenum.

manganese. silicon, titanium, niobium, vanadium, carbon, nitrogen, phosphorus, boron and copper.

In particular, an object of the Invention Is achieved by providing an austenitic stainless steel alloy with Improved resistance to thermal creep at high temperatures, consisting essentially of, by weight percent: from 16 to 18% nickel; from 14 to 16% chromium; from 2 to 3% molybdenum; from 1.5 to 2.5% manganese; from.1 to.4 silicon; from 0.2 to 0.4% titanium; from 0.1 to 0.2% niobium; from 0.1 to 0.5% vanadium; from 0.08 to 0.12% carbon; from 0.03% to 0.05% nitrogen; from 0.03 to 0.07% phosphorus; from 0.005 to 0.01% boron; from 1.5 to 3% copper; and the balance Iron.

BRIEF DESCRIPTI - ON OF THE DRAWINGS Fig. 1 Is a graph of creep strain versus time for various alloys treated at 170MPa at 7000C in the. mill-annealed condition, as received from commercial vendor.

Fig. 2 is a graph of creep stress versus rupture life of the subject alloy as well as several other alloys. All were tested in the mill annealed condition, as received from commercial vendors.

Fig. 3 is a graph of creep stress versus rupture life of the subject alloy and INCONEL 617 at 7606C.

DETAILED DESCRIPTION OF THE PREFERRED E14BODIRENTS

The improvements in the physical properties of austenitic stainless steel alloys comprising this invention result from modifying the composi tion of a previously developed alloy with minor compositional changes and, in particular,, the addition of a small amount of copper. It is dis tinguished from previous steels in that Ti, V and Rb are present together in a higher Mo containing steel; the alloy is lower in silicon and higher in C, N, and B than similar steels and these modifications are incorporated into the high Mo, Ti, V and Nb containing steels; Cu is intentionally added to this composition. The resulting alloy is characterized by high creep strength, low creep rates and long rupture life at 700-BOOOC in the mill-annealed condition; the alloying additions interact to produce fine dispersions of phosphide and MC phase particles in the matrix for strength; and a grain boundary precipitate of i mixture of carbides, such as M23C6. NC, MC, with minimal formation of inter metallic Laves and sigma phases for enhanced rupture life.

The alloy of the previously filed application, herein refered to as ADV.MOD.1, was modified from an earlier alloy developed for improved radiation resistance. These alloying element modifications were intended to produce fine dispersions of MC and phosphide phase particles in the matrix together with a combination of finer MC and coarser M23C6 carbides at the grain boundaries. The ADV.MOD.1 composition was further selected to prevent the formulation of intermetallic phases like Laves, chi or sigma that can cause embrittlement at grain boundaries. If formed during 6 irradiation, this microstructure should prevent void swelling and helium embrittlement. This portion of the previous invention Is currently being tested in various reactor experiments.

The present invention combines similar precipitate effects obtained from ADVA0D.1 with the addition of 2-3 wt% Cu to obtain the alloy of this invention designated ADMOD.2. Copper is added to alloys like 17-14 Cumo for its effect on improved solid solution strength properties of the alloy, but has not been noted previously to have any effect on precipitation is austenitic steels. The 17-14 CuMo alloy also has Ti and Nb additions that produce MC precipitation. Based on analytical electron microscopy data on the compositions of fine phase particles, Cu is not found to concentrate directly in the MC, phosphide or M23C6 phases produced in aged ADVA0DA, even though it is present in trace amounts in the alloy. These data, together with various other previous observations, led the submitters to believe that the Cu addition could be made to the ADVA0D.1 composition to superimpose its solid solution benefits without interfering with the desireable precipitate effects already achieved.

EXAMPLE

The alloys of the invention were prepared by two different commercial vendors using methods that produce residual element chemistries typical of larger scale commercial practice. This differs from typical experimental alloys, which are generally prepared under laboratory conditions in Inert atmospheres and may be deficient in important impurities compared to commercial practices (i.e., commercially prepared steels are exposed to oxygen and nitrogen.throughout the process). This factor enhances the 7 utility of the invention, as preparation of the alloys involves no special procedures or equipment. Likewise. the alloys may be fabricated into desired form by conventional techniques.

After the alloys were prepared, they were mill-annealed at about 120CC. Early creep data on a heat of ADGMOD.2 at 7000C and 170MPa, shown in Fig. 1 show a factorof four improvement in creep rupture life relative to a heat ADV.MOD.1 with exactly the same composition, but without the Cu contained in AH.MOD.2. AN.MOD.2 shows a factor of three improvement compared to 17-14 CuMo. Fig. 2 shows general improvement of rupture strength of AN.MOD.2 over a range of stresses at 7000C compared to Japanese Tempaloys -Al and -A2, and to CR30A. All these steels were tested in the mill-annealed condition. Finally, Fig. 3 shows that the creep rupture strength of AW.MOD.2 extends to higher temperatures and approaches the strength of INCONEL 617 at 7600C. This is important because if the maximum use temperature is increased then more reliable performance at lower temperatures can be expected. Strength advantages of ADV.MOD.2 may extend up to 8000C, and current research is aimed at testing this facet of its performance.

This alloy composition permits a tailored precipitate microstructure which develops in the mill-annealed material during service. This is important because ease of fabrication is greatly enhanced by not having to heavily cold work the material or subject it to complicated thermal mechanical treatments in order to develop strength and microstructure prior to service.

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Claims (1)

1. An austenitic stainless steel alloy, with improved resistance to thermal creep at high temperatures, consisting essentially of, by weight percent: from 16 to 18% nickel; from 14 to 16% chromium; from 2 to 3% molybdenum; from 1.5 to"2.5% manganese; from 0.01 to 0.4% silicon; from 0.2 to 0.4% titanium; from 0.1 to 0.2% niobium; from 0.1 to 0.5% vanadium; from 0.08 to 0.12% carbon; from 0.03% to 0.05% nitrogen; from 0.03 to 0.07% phosphorus; from 0.005 to 0.01% boron; from 1.5_to 3% copper; and the balance iron.

Published 1988 at The Patent Office, State House, 66.171 High Holborn, London WC1R 4TP. Firther copies may be obtained from The Patent office, Sel eS Rrancb, St Mary Cray. Orpington, Kent BR5 3RD. Printed by Multiplex techniques Itd, St Mary Cray, Kent. Con. 1/87.