A st ud y was made of t he action of boron in relation to the hardenability of high-purity alloys varying in carbon content. The effectiveness of boron in enhancing the harden ability of t hese alloys and certain steels is believed to be due to its action in retarding the rate of nucleation of ferri te and carbide while in soli d solution in austenite. The hardenabi li ty of t he boron-treated a ll oys, as determined in terms of the cri tical coolin g rate of small specimens a usteniti zed at a ,vide range of temperatures, varied with t he prior history and with the carbon content. The hardenabili ty of a commer cial boron-treated steel, as determined by the end-quench test, was also sensitive to prior thermal t reatments. Boron was lost in t he decarburized zone of commercial steels, and i ts rate of diffusion apparently is of t he same order of magni tude as that of car bon. The heat t reatment of specim en s of t he alloys and tcel s to produce a boron const it uent and the microstructures of t h e alloys as cast and as h o mo~e ni ze d are described .
A series of high-puri ty iron-chromium-nickel alloys, made to a base analysis of 18 percent of chromium and 10 percen t of nickel, and wi t h carbon rangin g from 0.007 to 0.30 percen t, was m elted and sol idified in vacuum . The presence or absence of carbides in t hese alloys after various m echani cal and t hermal t rea tm ents was determined by m etallographi c examinat ion . The practical limit of solubili ty varied from less t han 0.007 per cent of carbon at 1,300° F to sli ghtly m ore than 0.077 p ercent at 1,9 75° F , t he hi ghest temperat ure studied.The presence of ferri te and/or sigma a s stable phases in man y of t he alloys \"as est ablished . l. IntroductionThe austenitic stainless steels of AISI types 302 and 304 are essentially alloys of iron with 18 percen t of chromium and abou t 10 percen t of nickel, containing rela tively small amoun ts of carbon. Although the higher ca rbon co ntent of th e type 302 steel results in somewhat higher str ength than is obtained in its lower carbon counterpart, type 304 , it has long b een recognized that this higher carbon h as b een the cause of lowered corrosion resistance, particularly of th e intergranular variety. Th ese steels b ecom e susceptible to intergranular corrosion if exposed to temperatures tha t cau se the precipitation of chromium carbides. The use of type 304 stainless steel, with its 0.08 per cent maximum of carbon, was based upon the premise t hat th e lowered carbon content would render the steel more r esistant tointergranular corrosion, and this point of view has b een emphasized b y the r ecent d evelopment of th e type 304 ELO steels, with carbon h eld to a maximum limit of 0.030 percent.Even th ese extra low carbon steels, however , precipitate carbides under certain conditions of time and temperature. Although such steels are definitely less susceptible to intergranular corrosion than the regular type 304, it is ' apparent that complete immunity to inter granular corrosion of unstabilized austenitic stainless steels can b e achieved only wh en the carbon conten t is less than the limi t of solid solubility. It is now gener ally b elieved that the limit of solid solubility of carbon is less than the valu e of 0.02 p er cent usually assumed.The austenitic stainless steels are annealed with the obj ect of obtaining a completely austenitic stru cture with all carbon in solid solution. To attain this end, annealing temperatures used in commercial practice are quite high, u sually about 1,950 to 2,000° F. Although such high annealing temperatures are effective in causing the solution of precipitated carbides, the use of lower annealing temperatures with smaller resulting grain siz e is consider ed desirable from th e viewpoint of resistance to intergranular embrittlem en t . As th e temperature of complete olution of carbides is a function of carbon content, it 40 cJ appears logical to presume that the low-carbon steels \ may be annealed at lower temperatures than the higher carbon varieties. In the absence of defini te Imowledge of th...
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