A stainless steel alloy

Abstract: A chromium nickel silicon stainless steel alloy with improved wear resistance consists of, in weight percent, 19 to 22 chromium, 8.5 to 10.5 nickel, 5.25 to 5.75 silicon, 1.7 to 2.0 carbon, 8.0 to 9.0 niobium, 0.3 to 0.5 titanium and the balance iron plus impurities. The addition of titanium and increased amounts of niobium and silicon alter the microstructure of the stainless steel to form a duplex austenitic/ferritic microstructure which undergoes secondary hardening due to the formation of an iron silicon intermetallic phase.

Description: The present invention relates to a stainless steel alloy and in particular relates to a chromium nickel silicon stainless steel alloy that is especially suited for use as components in nuclear reactors, particularly in the components used in the steam generating plant of nuclear reactors. Currently cobalt based alloys are used in the steam generating plant of nuclear reactors, but cobalt has a long half life making the use of cobalt undesirable for use in such applications. Some known iron base alloys have good wear properties, but insufficient corrosion resistance. Some known nickel base alloys have good corrosion resistance but poor wear resistance. One known stainless steel potentially suitable for use in components of nuclear reactors is disclosed in GB-A- 2167088, and this comprises broadly speaking 15 to less than 25 wt% chromium, 5 to 15 wt% nickel, 2.7 to 5.5 wt% silicon, 1 to 3 wt% carbon, 5 to 15 wt% niobium plus vanadium, up to 0.15 wt% nitrogen, up to 1.5 wt% cobalt and the balance iron plus impurities. This alloy contains very little or no cobalt thus having a low half life. The particular stainless steel alloy available commercially from Deloro Stellite, St Louis, Missouri, USA under the trade name Tristelle 5183, comprises in weight percent 19-22 chromium, 8.5 to 10.5 nickel, 4.5 to 5.5 silicon, 6.5 to 7.5 niobium, 1.8 to 2.2 carbon, up to 0.1 nitrogen and balance iron plus impurities. The alloys suitable for use in steam generating plant of nuclear reactors must have high wear resistance and high corrosion resistance. The alloys disclosed in UK patent 2167088 have been tested and it has been found that they have a hardness of 350-450 Vickers (38-44 Rockwell C performed on a Rockwell hardness testing machine). The present invention seeks to provide a stainless steel alloy suitable for use in nuclear reactors which has greater hardness than the known stainless steel alloys. Accordingly the present invention provides a stainless steel alloy consisting of, in weight percent, 15 to 25 chromium, 5 to 15 nickel, 2.7 to 6.0 silicon, 1 to 3 carbon, 5 to 15 niobium, 0.3 to 0.5 titanium and the balance iron plus impurities. The most preferred stainless steel alloy consists of, in weight percent, 19 to 22 chromium, 8.5 to 10.5 nickel, 5.25 to 5.75 silicon, 1.7 to 2.0 carbon, 8.0 to 9.0 niobium, 0.3 to 0.5 titanium and the balance iron plus impurities. Preferably the alloy is hot isostatically pressed The alloy may be used for making articles or components or may be used for coating articles or components. The present invention will be more fully described by way of reference to the following example. The basic commercially available stainless steel sold under the trade name Tristelle 5183 was modified principally by the deliberate addition of titanium to the stainless steel alloy, and further modified by increasing the amounts of niobium and silicon present in the stainless steel alloy. In particular the titanium was added such that the stainless steel alloy consisted of 0.3 to 0.5 weight percent titanium, the niobium was increased such that the stainless steel alloy consisted of 8.0 to 9.0 weight percent niobium and the silicon was increased such that the stainless steel alloy consisted of 5.25 to 5.75 weight percent silicon. These controlled additions of titanium, niobium and silicon alter the structure of the stainless steel compared to that in the commercially available Tristelle 5183. The additions of titanium, niobium and silicon produce a duplex austenitic/ferritic microstructure which undergoes secondary hardening due to the formation of an iron silicon intermetallic phase which has been identified by electron transmission spectroscopy. Further hardening is achievable by hot isostatic pressing (HIPPING) of the stainless steel alloy in powder form. The stainless steel alloy of the present invention creates a duplex microstructure within which secondary hardening occurs. The secondary hardening only occurs in the ferrite phase. The actual stainless steel alloy consists of, in weight percent, 19-22 chromium, 8.5 to 10.5 nickel, 5.25 to 5.75 silicon, 1.7 to 2.0 carbon, 8.0 to 9.0 niobium, 0.3 to 0.5 titanium and the balance iron plus incidental impurities. The impurities may be up to 0.2 weight % cobalt, up to 0.5 weight % manganese, up to 0.3 weight % molybdenum, up to 0.03 weight % phosphor, up to 0.03 weight % sulphur, and up to 0.1 weight % nitrogen. The stainless steel alloy of the present invention has been prepared and tested and it has been found that it has a hardness of 475-525 Vickers. Thus it can be seen that the stainless steel alloy of the present invention is considerably harder than those of the prior art, making the stainless steel alloys of the present invention more suitable for use in nuclear reactor steam generating plant, or other applications where high wear resistance is required. The additions of titanium, niobium and silicon may also be applied to the broad stainless steel alloy range of GB-A- 2167088. The stainless steel alloy of the present invention may be used in the form of cast articles or components, in weldings or hard facing materials applied to articles or components, in wrought articles or components or in powder metallurgy articles or components.

Claims: A stainless steel alloy consisting of, in weight percent, 15 to 25 chromium, 5 to 15 nickel, 2.7 to 6.0 silicon, 1 to 3 carbon, 5 to 15 niobium, 0.3 to 0.5 titanium and the balance iron plus impurities. A stainless steel alloy as claimed in claim 1 consisting of 19 to 22 chromium, 8.5 to 10.5 nickel, 5.25 to 5.75 silicon, 1.7 to 2.0 carbon, 8.0 to 9.0 niobium, 0.3 to 0.5 titanium and the balance iron plus impurities. A stainless steel alloy as claimed in claim 1 or claim 2 wherein the alloy has been hot isostatically pressed. An article comprising a stainless steel alloy as claimed in any of claims 1 to 3. An article having a coating comprising a stainless steel alloy as claimed in any of claims 1 to 3.

Inventors: BURDETT WILLIAM BARRY, BURDETT, WILLIAM BARRY
Applicants: ROLLS ROYCE & ASS, ROLLS-ROYCE AND ASSOCIATES LIMITED
Application year: 1996
Main classification: C22C38
IPC codes: C22C38, C22C33