CA1065561A - Tool steel - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Reduced decarburization production of steel is achieved by the process provided herein. It includes the step of preliminarily cleaning and roughing the exterior of a steel workpiece, and then applying an adherent removable protective surface coating of metal material onto the exterior of a steel workpiece to be heated, e.g., by providing a metallized, e.g., aluminized, adherent removable protection surface coating on that cleaned surface of the steel workpiece, Then the process involves carrying out such heating of the hereby protected steel workpiece, such heating including at least one hot working step, e.g., by heating for hot-working and/or annealing, e.g., of the semi-finished billet. Then the resultant metal material coating is removed. Novel metallized coating products are thereby fabricated. Thus, by this invention, an efficient and economical process for heating tool steel substantially protected against attendent decarburization during such heating is provided. This avoids the problems of conventional heating techniques.

Description

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he present invention relates to and has amon~ its ¦l objectlv~s the prod~ction of steel~ wi~h the sub~tantial elimination or reduc~ion Of decarburization and more particularly concerns the efficient and economical substan~ial elimintation of decarburization during heating of the 5. ' s~eel by providing an adherent temporary protective surface , coating of metal material such as for ~xample~ aluminum applied to its exterior prior to such heating for working and anneali~g.

While the invention is applicable in the produc~ion o~
~ al1 steel having decarburization problems~ hereinafter solely 10. ¦' to fac;litate an understanding of the invention, it will be ¦! described in connection with the production of tool steel. It ¦ is to be understood that this description is in no way to be ¦I considered a limitation upon the invention which, as aforesaid, ,~ is pertinent to all steel in whose production decarburization is I' ~
15. !, a problem.
, ~n inherent result of heating steel to form semi-finished!
i or finished products is that some of the surface carbon content ¦
o thQ ingot, bloom, slab, billet~ bar, preforms, etc. being 1l worked is lost by oxidation due to the surrounding atmosphere.
20. ,I For example, in the instance of tool steel~ especially high speed 1` steel, this loss of surface carbon or decarburlæation is a signi ¦'~ icant problem since it represents a corresponding decrease in usable tool steel yield and a concomitant waste of energy at a time when energy conservation is of recognized importance~

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l. ¦ In the fabrication of tool steel products, steel ormulati~on ¦. melts such as, for example, those produced in conventional electric are ., furnaces are tapped into ladles and teemed or poured into molds of I' selective size ~o cast ingots. Each heat o ingots contains a ;5. Il specified carbon content as well as other alloying constituen~s i~ corresponding to the properti~es desired in the finished tool stee product. A~ter being stripped from their molds, the ingots are heatéd and subjected to a series of hot working and annealing ¦l manipulations to provide selectively shaped and sized tool steel ¦
¦ products which may then be finally thermally treated as desired. ¦
¦~ Tool steel ingots and¦or billets are also made by powdered ¦l metallurgy process wi~h similar alloying eleme~s including carbonl ¦l and are fabricated into semi-finished and finished products similar !! ~o the cast ingot product with similar problems of decarburization.
-ls. ! Therefore~ al1 references to cast products are equally ¦~ applicable to powdered techniques and such references are used fo~
con~enience only. ¦
li For instance, the cast ingot is conventionally heated t~ its ¦l hot-working tempera-ture, to permit it to undergo permanent 70~ deformation by the application of mechanical forces to ;ts surface, so as to obtain products of specific size and Il shape often with improved physical and mechanical properties.
However, these hot working manipulations such as hot pressing or hammering or blooming ,he ingot into billet l, form, hot rolling the billet in~o a reduced size workpiec~, I
, and finally hot rolling the workpiece into a semi-finished i

2 ~ ' l 1. I workpiece, followed-by annealing and/or heat treatment, all expose the workpiece to signi-ficant loss of surface carbon. ., I Consequently, after each oE these heating cycle .
¦ manipulations, the outer periphery o the workpiece becomes 5. 1 decarburized to ~pme extent. To achieve a tool s-teel ¦l product substantially free of decarburization, the w~rkpiece is usually ¦ ground a:Eter each hot working operation to 2 sufficient - depth to remove surface defects and the decarburized zones I or areas.
lOo . After annealing, the workpiece, e.g. in bar Eorm, is usually straightened by rolling on straigh~ening rollers and then subjected to final cold finishing operations such as, for example, peeling and/or centerless grinding to remove surface defectsl ¦

¦l and decarburization, if present. The finished worl~piece -~15. ~, or tool steel product thus represents only a part of the ~ I original quantity of tool steel, often amounting to yields -: , of only 50% by w~ight based on the starting melt. j .

Experience in particular with the fabrication of ¦
¦' hot rolled bar and like products of tool steel; especially ~ . 1~ high speed steels, indicates that loss of su~face carbon here it can adversely affe~t the quality of the finished .
product, occurs mainly at two stages o~ the processing~
namely during billet heating for finaL rolLing and during the final annealing cycle. Tool steel ~anuEaceurers over ¦

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1. ' the years have attempted to reduce the amount o~ decarburi-i zatiQn by ~arious means. Protective coatLngs have becnused to try to solve the problem but due to their drawbacks I in practice~ satisfactory performance at low cost has not , 5. 1~ been achieved.
¦~ Speci-Eically, one practice involved the use o~
¦ borax compound to coat bill~ets by dipping and brushing before hot rolling. This expedient was fairly successful , in protecting the workpiece against decarburization during 0. I heating for rolling; however, it did not provide an~ protec tion during annealing. Besides, the borax stop-off coating made the bille-ts very slippery to handle during rolling operations and this presented an unnecessarily dangerous ¦, situation for the mill personnel.
15. ¦~ Recently, other types o-f protective coatings and ¦
¦~ stop-off materials have been utilized with some degree of l success including graphite~ceramic type and metal-base paint coatings~ Ho~ever~ none of tkese coatings prDvide ¦I sufficiently effective surface pro,ection against decarburi 20. ¦I zation both during billet heating for rolling or during 1 annealing~
¦¦ A more drastic alternative adopted by some tool j , steel manu~accurers has been the abandonment of those tool I
steel manipulative steps designed to reduce decarburizati~n 'i j 1 :' i . I 1.

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1. 1 in favor o~ contro~led atmasphere or vacuu~ annealing !I fnrnace inst:allations. ~arge ca~ital expenditur~s have ¢
been made ~or con~rolL~d ~tmospl~ere ~llrnaccs for the re- _ 1 lleatill~ and llot working o~ the ~ool s~el ~70rkpleces an~ ~o~
5. 1~ vacuum anneal:ing ~urnaces and the like, ~llich l~ecause of ¦I their controlled ambient environments achieve tl~e desired ~
~¦ reduction of decarburization. The obvious dra~back o~ th~es r ¦ processing syst~ms is their high c~pital investment for , speciEic equipment and suksequent operating costs.
10. ¦ ~s a consequence, in connection with presen-t day 1 l ! ~ool steel manufacture, either protective coatings or stop-I o~f treatments or hi~ cost appara~us installations are ¦
¦ ~mployed, which entail the above-noted shortcomings~ or con-¦ ventional hea~ing techn~ques are utilized,without any 15. special decarburization minimizing precautions, and relying r :~ I on ~inal cold finishing operations suc~ as those involving ~r peeling and/or centerless grinding to remove xesulting t~
¦; decarburization, despite their offsett:ing overall low 1', yi~lds. ~
20. Il According to an aspect of the present invention, an efflcient ¦ and economical proc~ss for heating o~ tovl steel substan-1, tially prot~cted a~ainst attendant decar~uriza~ion during ¦I such heatlng is provided, In one of it~s broad aspects, such .
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process comprises preliminarily cleaning and roughening the steel workpiece exteriorly; appLying an adherent remDvc~ble protec-tive surEace coc~ting of metal material onto the exterior of a steel workpiece to be heated; carry-ing out such heating of the thereby protected steel workpiece, the heating including at least one hot working step; and thereafter removing the re-sultant metal material coating.
Advantageously, by one variant thereof, the workpiece is prelimin-arily clec~ned and roughened, by grit blasting, or some other means, to ~
enhance or increase the adherence of~the thereafter applied metal material aoating, and the metal material of the coating is suitably applied in an average thickness of 0.006-0.010 inch, for example by metal spraying. By !
another variant, the metal material of the coating may be a non ferrous metal, preferably aluminum.
Contemplated heatings include the usual hot working and annealing steps. The hot working step may appropriately include one or more hot rol-ling steps of the conventional type, e.g. t~ose carried out at the hot w3rking temperature of the tool steel, which is usually initially at least 1800F, and the same may be followed by conventional annealing. Removal of the resultant metal material coating and any attendant reduced decarburi-zation in the outer peripheral portion of the workpiece may be effected byconventional cold finishing operations such as peeling and/or centerless grinding.
~ ore particularly, in another aspect, the present invention pro-vides an overall process for heating of tool steel to form a product having a reduced decarburization outer peripheral portion. Such improved process comprises: hot pressing a tool steel ingot at its hot-working temperature - into billet form; cooling the resultant billet form workpiece; annealing and removing the outer peripheral portion thereof containing any surface defects; reheating the billet form workpiece to its hot w~rking temperature and ~ot rolling the workpiece to reduc~d size billet form; cooling the ~s~
reduced size billet form workpiec,~; anneali.ng and removing the outer peripheral portion thereof co~taining any surface defect~ and deca~butiza-tion; preliminarily cleam ng and roughening the exterior of the recluced size billet form w~rkpiece to enhance the adherence of the metal material coating to be thereafter applied; applying an adherent rem~vable protective surface coat.ing of metal material onto the exterior of the reduced size billet form workpiece after the prelimlnary cleam ng and roughening; reheating the re-sultant metal material coated workpiece to its hotJworking temperature and hot rolling the workpiece to finished size form; annealing and straighten-10 ing the finished size form workpiece~ and thereafter removing, from the finished size form workpiece, the resultant outer peripheral portion thereof containing any surface defects and decarburization and the resultant metal r material coating. Alternatively by a variant thereof, after annealing and straightening the finished slze form workpiece, heat treating the workpiece and then removing the resultant outer peripheral portion thereof containing any surface defects and decarburization and the resultant metal material coating.
By another variant, the process is carried out wherein the re-mDving of the outer pPripheral portion from the billet form wDrkpiece and ,-20 from the re~uced size billet form workpiece is carried out by grinding;
wherein the preliminary cleaning and roughening of the reduced size billet form wvrkpiece are carried out by grit blasting; and wherein the remDving of the resultant outer periFheral portion and the resultant metal coating from the finished size form ~orkpiece are carried out by cold finishing.
According to one preferred embodiment of the invention, a melt oonfirming to the constitution of a typical high speed steel or tool steel, such as, for example, that produced in an electric arc furnace, is cast into ingots. The ingots are reheated to their hot working temperatures, between 1700F and 2300F in the case of high speed steel, and hot worked to a . - 7 -`` ~LO~S56~
semi-finished condition~ m ese hq~ working steps include, the case of rolled bar production, the hot pressing or blooming on a rolliny mill -the heated ingot workpiece into billet form, the removal of the outer peripheral por-tion therefrom, e.g. by grinding or scarfing to elim mate surface defects, reheating the billet to its h~t working temperature again and hot rolling, pressing or ha~ering to the semi-finished redured size billet form, follcwed by the removal of the outer peripheral portion thereform thereby substanti-ally to eliminate surface defects and decarburization.
An adherent rem.ovable protective surface ooatinq of alumunum is applied onto the semi-finished workpiece exterior, and the alumlnum coated workpiece is r~heated to its hot workiny temperature and hot rolled to finished size. The workpiece is then annealed and straightened on straight- , eniny rollers, and the resultant reduced decarburization outer peripheral portion and aluminum coating are thereafter ,,~.. _ . , . . . _ ._ _.

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removed In ordeF to assure increased adherence of the alu~nu~ coating ~;
to the se~i~ini~shed workpiece~ the step o~ prel~minarily clean~ng and roughening the exteri~or surface thereo~ is ~ included prior to the aluminizing ~or the ~in~l hot rolling or working operations, i,e. .
by which the workpiece is ~abricated into its finished size for annealing.
It will oe realized that the metal materifll coating is advantageously applied at a point in the ~ool s~eel fabrication at r least as early as the production of the semifinished si~e billet form workpiece. This is because the subsequent operations, involving the reheating for final rolling and the final annealing, are those heating cycles where loss of surface carbon mainly occurs. However, in its broader aspects, the present inventioa also contemplates applying the ;
protective metal ~aterial coating at an earlier point such as prior to L
reheating for hot rolling to reduced size billet form where decarburization alsQ occurR.
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~y reason of the ren~vable nature of the protective metal . .
material coating, and the intended further successive heat~ng cycle r~
ho~ working operations, the present invention in jet another-aspec~ t~_~
~ also contemplates specific intermediate articles of ~anufacture. One of these is the unannealed and unstraightened semi-finished tool steel workpiece having an adherent removable protective surface coating of metal ma~erial, e.g. I

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g_ 1065Slil 1. , in the form of a metal sprayed a~lLminum coating in a thiclc- z . I ness of 0.006-0.010 inch, onto the exterior thereof 9 I preferabl~r with such exterior llaving been preliminariLy jl cleaned and roughened prior to applica~ion o~ the metal 5. ¦I material coating thereto. , The other of these is the tool steel finished ~orkpiece 'I having a reduced decarburization outer peripheral por-tion I and an adherent removable protective surface coating of ¦ metal material previously applied onto the exterior o the 10. ; precursor semi-finished workpiece, e.g. in an original ! thickness of 0.006-0.010 inch, and which semi-finished -¦~orkpiece has been subjected to hot working temperatures, - I e.g. including hot rolling at a temperature of initially at I! least 1700F to 2200F followed by annealing and 15. !l optional straightening, after the metzl material coatlng has I
¦, been applied thereto~ and subjecting the workpiece to addi j ~ tional heat treating temperatures o initially at least ,1 1500F to 2350F,whereby to for~ such heat treated tool l ! steel finished workp;ece~ In this instance also, the exterioF
20. ¦, of the precursor semi-finished workpiece preferably has j been preliminarily cleaned and roughened as stated.
- The adherent protective surface coating of metal ~¦ material such as, for example, aluminum may be applied onto the exterior I j i~ of the tool steel workpiecè by various convcntional ¦ ¦
25. metallizing techniques, such as, for example, by metal spraying.
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,, '. ,/' Metallizing by aluminum spraying is used for e-~ficient i covering o the workpiece at a conveniently controllable ! thickness. .
¦ Generally~ metaL spraying invol~es the heating of 5. ¦ the metal to be sprayed to rnolten or semi-molten condition by passage through a high'tempera-ture zone, and the de-¦ positing of the sprayed metal in a finely divided form onto ¦ the sur~ace of the article to be sprayed. The moltert or I semi-molten particles of the sprayed metal flatten out on 10. ¦ impac~ with the substra~e sur:Eace being sprayed and adhere ¦ thereto upon -freezing. Subsequently deposited particles ¦ will also ~latten out and adhere in turn to those previousl~ , I deposited to provide an incrementally built-up structure o~
i sprayed deposits which is lamellar in ~orm.
15. I The metal to be sprayed is often supplied in wire li or powder ~orm. Thus~ w~en metal in wire form is rendered ¦~ molten, it can be subjected to a high velocity blast o~ l air or other gas to atomize and propel -it onto the subs-trate surface. Various metallizing guns or s~milar apparatus are¦
20. ~, avaiiable to spray wire, rod, or powder and they commonly , utilize a mixture of oxygen and acetylene or o-th~ similar ¦~i gases as the heat source. Arc spraying guns are also used ~ hereby wire is melted in a high heat zone resulting from a ,~.
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1Q1655~1 .C. Arc and the molten particles are swept off at a h-Lgh velocity by compressecl air.
¦, Advantages of using the rnetal spraying technique to achieve the desirèd thickness co~-ting are that it is not1 , 1' limited to any particular si~e of workpiece, and the availa~le ¦ metallizing guns are handy ~o use.
¦ In the case of aluminum as well as other conven ~I tionally sprayed metals, such as, for example, the non-fe~rous metals:
¦I copperj bronze, lead, molybden~, nickel, tin and zinc, and lO. ¦l even low carbon~ high carbon and stainless steels, the sprayed metal deposits resemble chemically the derivative wire, rod or powder but their physical properties are general~y quite different from those of the metal before , spraying. The sprayed metal deposits provide a lamellar 15. ,I structure whlch is not homogeneous and cohesion is due to il mechanical bonding. Nevertheless, for the reduced decc~rbur7-zation protective surface coa~ing purposes of the present 1'l invention, the metal spraying technique is quite adequate jl as the means for applying the desired coating layer.
~20. ~I To enhance the adherence of the metal materîal , coating to the workpiece, as aforesaid, a cleaning and j' roughening step is preferably included. Thus, befor~ the j ¦~ metal sprayed coating is applied, the surface of the work- 1 piece is cleaned and prepared in a manner which will provide 1. I, i.

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a good bonding of the sprayed metal particles to the base l metal. The cleaning operation contemplates removal of greas e, I scale, dirt, oil and any o~her con~aminan-ts that would ;mpaiF , I the bonding of the coating. Roughening of the sllrface of thf 5. ~ workpiece is -the final operation prior to metal spraying.
i Conventional mechanical ro~ughening techniques are similarly employable to accomplish the desired purpose. As will be I appreciated, both cleanness and roughness afect greatly th ¦ bond strength between the metal coating and ~he substrate 10. I surface of the workpiece.

An advantageous combination expedient for achieving simultaneously both cleaning and roughening of the workpiec~
exterior surace is the use of a co~entional grit blasting - ~ step. Abrasives commonly used for preparing th~ surace in 15. I this respect are crushed angular sand, crushed steel grit ¦
and aluminum oxide. Steel grit or aluminum oxide is pre-ferred since the abrasive can be readily reclaimed and reused.
; The workpiece should be metallized or coated as soon j as possible after the cleaning and roughening operations, ¦
20. I in order to minimize surface oxidation and recontamination~ ¦
¦I Thus, for instance, where a part has been grit blasted and is to be metal sprayed, the workpiece may be immediately ¦
subjected to metal spraying, the relative movement of the workpiece and rnetallizing gun being regulated mechznically ¦

~ 6 l. 11 to the extent possible to insure uniEormity and repeatability .

j It has been found ~hat where the surface coating bein~ sprayed onto the workp;ece exterior is built up to a ~ thickness of bet~een 0.006-0.010 inch over the substrate 5- ~l sur~ce, a satis~actory protection is insured during heating , ¦l for rolling and final annealing. Thicknesses belo~
¦1 0.006 inch are generally insufficiently thin to work proper-¦l ly whereas those above 0.010 inch are unnecessary.

It is believed that the protection obtained during the heatin g 10. cycle before rolling is due to the mechanical bonding of the applied coating to the workpiece exterior sur~ace while the ¦ protection for annealing results from a metallurgical welding ¦l and dif~usion occurring between the resultant oxides of the I¦ applied metal material coating and the substrate as a 15. Il consequence of the hot working.

!i The protection against decarburization provided by the process of aspects of the present invention is unique in that it affords I

j ~I such protection not only during heating, e.g. of billets, ¦
~I for rolling, but also during the annealing cycle. Such pro-20. I, tection may, if desired, continue to be effective during a j subsequent heating cycle for hardening, i.e. prior to removal ,1 of the applied protective coating, such as, for example, in the i' manufacture o~ tool bits from stock rolled and annealed : 11 . l 1l - 14 _ , I ~
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il ~0655~ 1 , 1. ¦1 in the instant manner. ~dvantageously, ultimate protectionj against normal degrees of decarburization is thereby made ¦
possible during two or three consecutive and important I
¦ heak:ing cycles or working operations, by providing a l 5. , metallized surface coating of the type descr-ibed in I¦ accordance with the present inventiGn.
¦¦ There would no-~ appear to be any other coat-ing ¦ process so ar known that is able to provide this degree of I protection against decarburization from a single coating 10. I application. Moreover, the metallizing contemplated is particularly effective when aluminum is utilized as the spe-cific metal material applied as the adherent temporary protec lve surface coating on the workpiece, since it most I
¦ ef~iciently becomes a part of the substrate and affords more lS I effective protection ' It will be rea~ized that while broadly the meta~
material coating may be considered a coating of metal, i~ ¦
also contemplates the presence of some of the metal ;n the !
form of oxides. For instance, as is kno~, al~inum readily ~20. i converts at its exposed surface to ;~s oxide. Thus, when ;~ ~ sprayed from a metallizi~g gun, the molten atomized parti-¦ cles will readily convert to the oxide at their periphery , yet the overall result will be an aluminum-predomina-ting !' metal layer fully protective of the workpiece surface as ¦
25. ¦i regards decarburization.

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Metallizing of the billet or workpiece is satisfactory, especially considering the fact that the metal coating is thereafter sub~bcted to repeated heating cycles and hot working operations often above its melting temperature yet ; below its boiling temperature. Heating and hot rolling assures flow distribution of the metallized coating and achievement thereby of a sufEiciently uniform and relatively non-porous, skin, e.g. oE aluminum and/or aluminum-aluminum oxide, covering over the workpiece for protection against ox:Ldation and decarburization during annealing. This skin will provide an effective mechanical bonding initially to the workpiece substrate and as a result of the mechanical hot working operation, eventually also a metallurgical welding and diffusion.
As compared with processes involving controlled atmosphere installations to assure reduced decarburization during billet heating for hot working and/or annealing or other thermal treatments, the order of magnitude of present day in-vestment for the same capacity is $15,000 to $70,000 for the instant process in its various aspects depending on the degree of mechanization, and $500,000 to $750,000 for either the controlled atmosphere furnace or the vacuum annealing furnace installation type process. Thus, the present invention in its various aspects may be practiced at a mere fraction (1/33 to 1/50) of the current cost of the con-ventional controlled environment production operation. It is completely surprising that despite present day trends toward relatively expensive controlled atmosphere or vacuum ~LC?655~
~i installations, the present invention in its variQus aspects provide a more , efficient alternative at a correspondingly lower cost ln terms of yield and energy~
!, The follo~ing c~amples are set forth by way o:E illus~ra~
5~ ~, tion. I
In the manu~acture oi a .544" to "547t' diameter round bar !
j' of hig~ speed steel from a melt having the follo~ing compo-¦l sition:
¦, Carbon 1.00%
10. il Molybdenum 8.75 Tungsten 1.75 Chromium 3 75 Vanadium 2.10

3, The-following steps are carried out to provide a semi-15. ~ finished billet workpiece: `
I 1. Melt charge in electric arc furnace !l 2. Cast into 12" ingot mold 1l 3. Heat ingot to forging temperature of 2100F
! 4. Press into 6" square billet or other specified 20. 1~, size.
¦ 5~ ~rind to remove surface defects.
Il, 6~ Reheat to specified temperature for rollin~, '` 2080~F
Il 7. Roll to 1-3/8" s~uare billet (semifinished) 250 li¦ 8. Fully grind to remove decarburization and surface Il defects.
¦l In the fabrication of the semi-finished billet ~ork-¦!Piece into the finished .544" - .547" round har, the 1~ following further steps are carried out under the conventional 30. ~I!process A and separately under one aspect of the invention process B: ¦

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il l ~ SS~ ~` , ' ' One aspect of the i Convent onal Process A Inven ion -ocess B

1. I 9A. Reheat.to specified 9B. Grit blcls~ to clean and temperature, 2100F ~ roughen workpiece ex-terior surface, (1:1 mix I lOA.~oll to 5/8" Rd. ture of G25 and G40) steel 5. To finish .544"/.547" Rd. grit. ¦ ll ¦ l~A.Anneal lOB. ~luminize by metal spray! ~'1 ing to provide a protec-l l~A.Straighten tive surface coating in ¦ , an average thickness of lOo 13A.Cold finish by peeling to about 0.008'l ! .556/.558" and centerless~ Wire s;ze: 3ll6" Dia j g~inding to .544~ 547" Rd.
to remove decarburizatiOn~ llB--Reheat to specified I temperature.

15. ¦ 12B. Roll to 19/32" Rd. bar - ~o finish: .544/.547"Rd~
¦ 13B. Anneal - 14B. Straighten -. . .
~ 15B. Cold finish by peeling t~ ~
20. I - - - .556/.558" Rd. and center-- - - - less grinding to .544l - .547" Rd. to remove from aluminum surface coa~in~
¦ The size of mold used and the pressing, heating, grinding 25. ~and rolling practices may be varied according to the inishedl product desired and the equipment available for processing. ¦ .
Based on the origina~ melt charge, the yield of tool steel ater cold finishing step 13A of the conventional process .
¦is only 50%, whereas the yield after cold finishing 3~. j step 15B of one aspect of the invention process B is 60% which ..
tests show to be approximately the corresponding yield after straightening the bar per conventional process step .
Ii 12A and before removing decarburization per step 13A. ~he j,60% yield according to one asp~ct of the inYention after removing the '! 1-!~ -18_ I

~0~55~1 l . , aluminum sur:Eace coating per step 15B represents a 20~ ' Il increase over the corresponding SO~/O yield according to the ¦ conventional process and in turn a 20% relative reduc-tion in decarburization. Based on the melt charge, approxi~
5. mately 10% rnore usable tool steel is provided according to .
. I an aspect of the invention which represents an overall savings at current ¦

I . I costs of roughly 8 to 15 cents per pound or 160 to 300 1 .
I dollars per ~on of total steel produced.
¦` In this example? the metal spraying was carried out 10. ~ j with an oxyacetylene gas-compressed air metallizing gun using aluminu~ me=al wire in the conventional manner.

~¦ A cross-sectional photomicrograph (200x) of the I resultant metallized 0.794'l round rod, before removing l 15. ~1l decarburization by peeling and centerless grinding~ exhibited, after a 5% Nital etch, a metallized surface layer of aluminu~
to a depth of 0.008" fully welded and diffused with and , mechanically bonded to the workpiece substr~te. The absence of any decarburization beneath the metallized layer could be 20; jl clearly noted. ¦
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i Example 1 is repeated except that in this case ¦ ¦
I ! in an aspect of invention process B the grit blast and alumi.nizing steps I . I
i are performed earlier in the operation on the 6" square I ¦
billet after grinding per step 5 and before reheating per I -~! 19~ ! 1 !

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step 6 for a,ch~eyi~ng co~pa.ra~ble ~esult~ to those of Example 1.

rt will be appreciated that the fore~oing specification and examples are set forth b~ way of ~llustration~ The reference to ~'tool steell~ is for illustrative purposes only as the inventîon is applicable in the production of any steel in which decarburization is a problem.

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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Process for heating steel substantially protected against attendant decarburization during such heating which comprises: preliminarily cleaning and roughening the steel workpiece exteriorly; applying an adherent removable protective surface coating of metal material onto the exterior of a steel workpiece to be heated; carrying out such heating of the thereby protected steel workpiece, said heating including at least one hot working step; and thereafter removing the resultant metal material coating.

2. Process according to claim 1 wherein said workpiece exterior is preliminarily cleaned and roughened by grit blasting or some other means to enhance the adherence of the thereafter applied metal material coating.

3. Process according to claim 1 wherein the metal material of said coating is a non-ferrous metal.

4. Process according to claim 1 wherein the metal material of said coating is aluminum.

5. Process according to claims 1, 2 or 4 wherein the metal ma-terial coating is applied by metal spraying.

6. Process according to claims 1, 2 or 4 wherein the metal material coating is applied by metal spraying in an average thickness of 0.006 - 0.010.

7. Process according to claims 1, 2 or 4 wherein the at least one hot working step is a hot rolling step.

8. Process according to claims 1, 2 or 4 wherein the at least one hot working step is a hot rolling step and further wherein the metal material coating is applied in an average thickness of 0.006 - 0.010.

9. Process for heating of steel to worm a product having a re-duced decarburization outer peripheral portion which comprises: hot pressing a steel ingot at its hot working temperature into billet form; cooling the resultant billet form workpiece and removing the outer peripheral portion thereof containing any surface defects; reheating the billet form workpiece to its hot working temperature and hot rolling the workpiece to reduced size billet form; cooling the reduced size billet form workpiece and removing the outer peripheral portion thereof containing any surface defects and decar-burization; preliminarily cleaning and roughening the exterior of the re-duced size billet form workpiece to enhance the adherence of the metal mater-ial coating to be thereafter applied; applying an adherent temporary pro-tective surface coating of metal onto the exterior of the reduced size bil-let form workpiece after said preliminary cleaning and roughening; reheating the resultant metal material coated workpiece to its hot working temperature and hot rolling the workpiece to finished size form; annealing and straight-ening the finished size form workpiece; and thereafter removing, from the finished size form workpiece, the resultant outer peripheral portion thereof containing any surface defects and decarburization and the resultant metal coating.

10. Process according to claim 9 wherein the removing of the outer peripheral portion from the billet form workpiece and from the reduced size billet form workpiece is carried out by grinding; wherein the preliminary cleaning and roughening of the reduced size billet form workpiece are carried out by grit blasting; and wherein the removing of the resultant outer peripheral portion and the resultant metal coating from the finished size form workpiece are carried out by cold finishing.

11. Process according to claim 10 wherein said hot working tempera-ture is initially at least 1700°F.

12. Process according to claim 11 wherein the metal material of said coating is aluminum; and wherein the metal material coating is applied by metal spraying.

13. Process according to claim 12 wherein the metal material coat-ing is applied in an average thickness of 0.006 - 0.010 inch.