US3282746A

US3282746A - Method of hardening wear surfaces and product

Info

Publication number: US3282746A
Application number: US325838A
Authority: US
Inventors: Thaddeus F Zlotek; Edward J Vargo
Original assignee: Formsprag Co
Current assignee: Formsprag Co
Priority date: 1963-11-18
Filing date: 1963-11-18
Publication date: 1966-11-01
Anticipated expiration: 1983-11-01
Also published as: GB1077219A; DE1521237B1

Description

1966 1- F. ZLOTEK ETAL 3,

METHOD OF HARDENING WEAR SURFACES AND PRODUCT Filed Nov. 18, 1963 INVENTORS THADD EU S F. ZLOTE ATTO R NEYS rats The present invention relates to an improved method of heat treating and hardening wear surfaces, and to the product of such a method. The product is herein typically shown and referred to as a high carbon steel or steel alloy sprag to be used as a wedging strut between concentric races of an overrunning clutch, and more especially a clutch subject to very high speed overruns. However, while a hardened sprag furnishes a good example of the advantages of the invention in point of method and product, it is to be understood that the invention is not at all limited to this particular type of production. Since the result of the method is to provide a chromized type of external surface layer or case which is extremely hard and wear resistant (substantially more so than a conventionally chromized case), the method is well adapted to the production of high speed bearing races or surface plates, rollers and ramps of a roller type overrunning clutch, and other usages involving high pressure and/or relative velocity factors.

Generally considered, the invention provides a method in which the sprag or other object to be treated is, in preparation for a subsequent and known type of chromizing or chromallizing, surface hardening treatment, subjected to a nitriding or carbonitriding heat treat operation; and in which the subsequently chromized product (after a usually necessary restoration of its body hardness because of annealing occasioned in chromizing) possesses a surface hardness and wearability characteristic far superior to that found in products conventionally treated by a known chromium diffusion procedure.

More specifically, an object and result of a combined preparatory nitriding in an ammonia-laden atmosphere, or following any other suitable known procedure for producing nitrides, and subsequent chrornizing is the provision of a surface layer or case on the object which has all the advantages of the conventional chromium carbide case over one produced by electrodeposition or other film or coating depositing operation, in regard to its integral bond with the body base metal, its being proof against peeling, spelling or flaking, its resistance to corrosion and abrasion, etc. Yet it differs materially from the result of plain chromizing in that the diffused chromium layer or case itself possesses an intermediate chromium nitride stratum or phase. This intermediate phase (thickness-wise) in the otherwise essentially chromized layer, as the result of the initial heat treatment in an ammonia atmosphere, imparts an extreme degree of surface hardness and wearability to the object, especially well suiting the method to the sprag production mentioned above, for certain speed and load operating conditions.

Conventional chromizing or chromallizing procedure as practiced today are contemplated for that phase of. the present method. One of these is a powder pack chrome diffusion or infusion operation in which iron or steel pieces to be treated are packed in a powder containing a ohromium hydro'halide refractory diluent composition effective upon heating in a furnace to not only produce the chrome diffusion but also to purge the furnace of air and maintain it air-free throughout the chromizing treatment. The furnace is heated to an elevated temperature which will be a function of the time duration of the treatment, and when gaseous diffusion has been continued sufficiently long to produce the desired thickness aterrt of layer or case, theparts are permitted to cool slowly. The resulting annealing effect ordinarily requires a later restorative heat treatment to re-harden the body of the part, but this is without effect on the chromized layer. The patent to Samuel No. 2,536,774 of January 2, 1951 relates to a chromizing treatment of this sort, in which the chromium carbide takes an integral bond with the ferrous or alloyed ferrous base metal. Samuel No. 2- 962,391 of November 2-9, 1960 is another.

Another more recent development in the chromiz-ing procedure is illustrated and described in the patent to Samuel et al. No. 3,021,321 of February 13, 1962. In this method chromium is diffused into the treated object from a source, as distinguished from a powder pack, having a predetermined chromium-halide concentration. The metallurgical effect is essentially the same as in the powder pack method. Samuel 2,962,391, disclosing the use of various hydrozine hydroh-alides, mentions the suitability of both methods. A chromium diffusible composition may also be coated on the object.

As indicated, either of these chromizing procedures may be employed in the second chromizing phase of the present improved method of the invention; and it is also to be understood that other known ehromizing procedures not incompatible with the objectives may be resorted to.

A specific objective of the invention is to provide a method in which a steel or alloy steel having a high carbon content of above 0.4%, such as SAE 52100, a chrome alloy, is the material of the sprag or other object to be treated, since a carbon content of less than this concentration has been found to lead to increasing ductility of the case after chromizing, rather than hardness. Such product is, following through cleaning, heat treated to introduce the nitride component, and, following certain incidental operations (such as tumbling to remove burrs, grinding to provide desired formations, and the like) is chromized to produce a surface layer or case of the desired depth, say 0.0003". It is then reh-ardened to restore its body or core to desired hardness for its uprpose.

It is notable that whereas a sprag of given metallic composition, as chromized without the preparatory nitriding or carbonitriding treatment, exhibits a surface hardness of 12001300 VPN, a similar sprag composition preliminarily nitrided or carbonitrided, then chromized and re-hardened under the same chromizing and rehardening specifications, has a surface hardness of well over 1500 VPN. This we attribute to the internal chromium-nitride modification of the primary chromized external layer at the intermediate phase referred to above.

The foregoing as well as other objects will become more apparent as this description proceeds, especially when considered in connection with the accompanying drawing illustrating the invention, wherein:

The figure is a microphotograph of a nital etched section of an SAE 52100 alloy steel sprag at a magnification of 2000 X.

In general, the combined nitriding-chromizing method of the invention, a specific embodiment or adaptation of of which will be hereinafter described in greater detail, is as follows:

A. Using as an example an extruded sprag stock of high carbon steel or alloy steel, the stock is cut to the desired sprag lengths, which are thoroughly cleaned to eliminate all surface contamination, and are heated in a furnace at a sufiicient temperature (200 F.-250 F.) to drive off all moisture present.

B. The sprags are placed in baskets in a not excessive depth and are placed in a heat treating furnace, such as an Ipsen type 400 or 800, the furnace having been properly stabilized in temperature before loading. They are then heat treated under a specification calling for the presence of raw ammonia, or any other means of introducing nitrogen into the steel, at a temperature in the general range of 1535 F.1550 F., for a soaking time at temperature depending upon the desired body hardness characteristic of the steel, for example 90 to 100 minutes.

C. Quenching and further cooling follow under controlled conditions, and drawing to relieve stress.

D. The sprags or other products are then conventionally chromized, as described in either of the patents to Samuel et al. identified above, or any other known chromizing or chromium diffusion procedure not inconsistent or incompatible with the objectives of the present invention.

E. Hardening follows to restore body hardness of the object lost by annealing in the chromizing treatment which, per se, is to be understood not to be a contribution of the present invention, except as performed subsequent to nitriding, as described.

The above steps of course have interspersed therewith and/ or precedent or subsequent thereto, as in the production of sprags and other special objects, certain steps or manipulations not bearing upon the positive heat treat aspects of the invention, such as tumbling to remove burrs or to polish, grinding to produce notches or to finish (other than at a working surface), inspection, and the like. These will be referred to in the detailed production specification.

It is contemplated that the nitride generating setting in which the preparatory heat treating is performed may also be a carbon-heavy atmosphere, such as to produce a concentration of carbide nodules adjacent to a chromized surface case or layer of the finished product. This ordinarily might be considered to be a poor heat treating practice, but it may be desirable in some instances in the present comprehensive nitriding-chromizing method, for such carbide inner layer forms a hard crust to support stably the chromized layer modified by the intermediate chromium nitride phase referred to above.

Referring to the figure, the reference numeral generally designates the base, body or core of the typical SAE 52100 steel instanced above. With a 1.0% carbon content, and as treated in a furnace atmosphere of high carbon potential of, say, 1.5% C, the core 10 exhibits a relative multiplicity of complex iron chromium carbide particles or nodules 12 in a fairly close concentration adjacent the external, modified chromized layer or case 14, to be described, extending therefrom into the steel core. These have the effect of affording an inner nodular supporting crust for the last named layer which is referred to above. However, if preliminary nitriding is performed in a relatively carbon-neutral potential, the occurrence of such ferrous carbide particles is less pronounced. Such would be the situation were the carbon potential of the heat treating atmosphere reduced below 1%, the carbon content of the steel in question, and with no differential causing carbon to migrate to the steel structure.

The modified chromized layer, generally designated 14, includes a basic iron chromium carbide solid solution 16, traversed along its approximate midpoint by the intermediate chromium nitride complex layer 18, which has a distinctly contracting, darker appearance, in a pink shade, as appears in a color microphotograph of the nitaletched section. The reference numeral simply designates a plastic mount for the section of the figure, employed in the polishing and etching of the surface. Whatever the mechanical effect of this intermediate nitrided interlayer or phase may be, its effect on the overall chromized layer case 14 is to substantially increase its hardness and wearability, particularly with the inner core strengthened by the iron-chromium carbide complex 12.

The following is a typical production heat treat specification for SAE 52100 sprags, including incidental mechanical and inspectional steps not necessarily called for at all in other types of production:

(a) Cut to length, cleaned of surface contamination and heated at 200 F.250 F. to drive off all moisture.

(b) Placed in open basket (Ipsen type recommended) 2- in depth not substantially exceeding 1 /2" for sufficient heat soak and subsequent thorough quench. Place in heat treat furnace (Ipsen 400 or 800 type) after conditioning same as to temperature and atmosphere.

(c) If run at high heat previously, furnace temperature should be brought down for /2 hour and stabilized before lOading sprags. Heat treating temperature to be maintained above the lower critical temperature, for instance 1535 F.l550 F. Dew point of atmosphere maintained at 10 :1 point. Atmosphere to consist of endothermic gas (200-225 cubic feet per hour), raw natural gas (40 cubic feet per hour or more) and raw ammonia (9 cubic feet per hour :1 cubic foot).

(d) Soak at temperature 10() minutes and quench in oil (Park type AAA) at 90 F. minimum to 130 F. maximum, and air cooled slowly in circulating air.

(e) Draw, after standing at room temperature for 3-4 hours, at 325 F.-350 F. to stress relieve only (for loads less than pounds a minimum of 2 hours).

(f) Inspect for hardness, tumble to remove burrs, grind to notch, tumble to'remove burrs for chromizing treatment and/ or inspect for surface finish.

(g) Chromize to 0.0003" case.

(h) Inspect as to depth of nitride-modified chromized depth.

(i) Harden to correct chromize-induced anneal.

(j) Inspect for modified chromize depth and hardness, tumble, and inspect surface finish, final dimensions, and magnaflux structure.

In regard to the condition of the furnace at the commencement of nitride heat treating, it need not be cleared of a carbon-heavy atmosphere at this time, and it is in fact desirable in certain instances to process immediately after a carburizing operation in the furnace, thereby lessening the required furnace time under certain conditions. If the sprags are processed in a relatively neutral atmospheric condition from a prior heat, more time in the furnace will be required for proper processing. A dew point of +25 or lower will maintain a carbon potential of 1.00% in the furnace atmosphere, and as indicated at (c) above, it is desirable in a commercial production treatment to maintain the dew point at approximately 10. The dew point should be checked before the work is placed in the furnace and before removal. Not in substantial excess of 100 pounds loading of sprags should be had in a single basket.

The quench oil should be Within the 90 F. F. temperature range before quenching, and proper control should be maintained during the quench so that F. is not exceeded. Air cooling should commence only after the complete mass reaches a stabilized oil temperature below 150 F., and air cooling should continue down to 75 F.

Room temperature having been reached, the nitride heat treated sprags should have a Rockwell hardness of 93 to 94 on the 15N scale.

Item (d) states the temperature soak time for a hardened steel. However, should it be desired to produce an annealed steel, the soak time may be reduced under the same conditions; and this duration is also acceptable for producing hardened steel sprags, provided the furnace is in carburizing condition from a previous load just prior to treating another.

In regard to the restorative hardening heat treatment (i) after chromizing, this may be done under any suitable specification, but in general a specification involving conditions similar to those of the nitriding or carbonitridmg heat treatment. That is, the sprags are cleaned of foreign surface contamination, are heated to drive off moisture, are heat treated in similarly limited load weights and depths, but in a furnace cleared of all heavy carbon or ammonia atmosphere. Typical conditions of temperature and atmosphere: Dew point35 to 40, endothermic generator-35 :5 points (no ammonia) at 1525 F.1550 F. for a minimum of 60 minutes, similarly quenched in oil, air cooled and drawn at 300 F.320 F.

The fully and finally treated sprag will exhibit the following physical properties:

Core hardness60 minimum Rockwell C, 90 minimum Rockwell N;

Surface hardness (measured flat surface)Knoop hardness 1350 with 100 gram load;

On micro-structure examination, an intermediate phase 18 of thickness approximately 20% of total thickness of modified chromized case 14 and complex nodular carbides proceeding immediately below case 14 into steel of core.

The chromium nitride modified case or layer 14 has, characteristically of a chromium diffusion layer, an integral bond with the base steel such as prevents peeling, chipping or flaking apt to occur in electrodeposited films and the like. It is extremely hard and wear resistant for the purposes of the present invention, but may, if a steel less than 0.4% carbon is treated, have a greater degree of ductility suiting it for other applications. It is resistant to corrosion and abrasion. In regard to maintenance of dimensional stability the method has the same advantage as before, and variation is readily predictable.

The nature of the nodular crust concentration at 12 is believed to be an iron chromium carbide complex, which may come from the components of the particular alloy itself, or entirely or in lesser part from the chromized layer 14.

The initial heat treatment in a nitrogen including atmosphere, or in a carburizing nitriding atmosphere, involves little more than is involved in conventional carburizing heat treatment, so that the cost of the combined nitridingchromizing operation approximates the overall cost of heat treatment of a product lacking the preparatory nitriding phase. In other respects, the improved method and product possess all of the advantages attributable to a simple chromizing procedure, plus those special and notable advantages in respect to hardness which stem from the intermediate, chromium nitride phase of the modified chromium carbide layer.

What we claim as our invention is:

1. A method for the surface hardening treatment of ferrous articles, comprising subjecting the article to a nitriding heat treatment, quenching the article and then subjecting the article to a chromizing heat treatment to produce a chromized external layer on the article modified by a chromium nitride phase.

2. A method for the surface hardening treatment of ferrous articles, comprising subjecting the article to a nitriding heat treatment at a temperature above the lower critical temperature, and then subjecting the article to a chromizing heat treatment to produce a chromized external layer on the article modified by a chromium nitride phase.

3. A method for the surface hardening treatment of ferrous articles, comprising subjecting the article to a nitriding heat treatment in an atmosphere including nitrogen and carbon, quenching the article, and then subjecting the article to a chromizing heat treatment to produce a chromized external layer on the article modified by a chromium nitride phase.

4. A method for the surface hardening treatment of ferrous articles, comprising subjecting the article to a nitriding heat treatment in an atmosphere including nitrogen and carbon in a percentage content of the latter at least as great as that of the article, and then subjecting the article to a chromizing heat treatment to produce a chromized external layer on the article modified by a chromized nitride phase, with an iron-chromium carbide layer adjacent said layer extending into the core of the article.

5. A method for the surface hardening treatment of ferrous articles, comprising subjecting the article to a nitriding heat treatment in an atmosphere including nitrogen and carbon in a percentage content of the latter at least as great as that of the article, quenching the article, and then subjecting the article to a chromizing heat treatment to produce a chromized external layer on the article modified by a chromium nitride hase intermediate the thickness of said layer, with a ferrous carbide-rich layer adjacent said layer extending into the core of the article.

6. A method for the surface hardening treatment of ferrous articles, comprising subjecting the article to a nitriding heat treatment in a high carbon potential and nitride producing atmosphere, quenching the article, subjecting the article to a chromizing heat treatment to produce a chromized external layer on the article modified by a chromium nitride phase intermediate the thickness of said layer, and heat treating the article to restore the hardness of said core as annealed by the chromizing treatment.

7. A method for the surface hardening treatment of ferrous articles, comprising subjecting the article to a nitriding heat treatment at a temperature above the lower critical temperature in an atmosphere including nitrogen and carbon in a percentage content of the latter at least as great as that of the article, quenching the article, subjecting the article to a chromizing heat treatment to produce a chromized external layer on the article modified by a chromium nitride phase intermediate the thickness of said layer, with an iron-chromium carbide layer adjacent said layer extending into the core of the article, and heat treating the article to restore the hardness of said core as annealed by the chromizing treatment.

8. A wear-resistant article having a ferrous metal body provided with an integrally bonded external chromized case modified by a phase of chromium nitride.

9. A wear-resistant article having a steel body provided with an integrally bonded external diffusion case modified by a phase of chromium nitride intermediate the thickness of said case.

10. A wear-resistant article having an alloy steel body provided with an integrally bonded external diffusion case modified by a phase of chromium nitride.

11. A wear-resistant article having a ferrous metal body provided with an integrally bond-ed external diffusion case modified by a phase of chromium nitride intermediate the thickness of said case, with a ferrous carbide-rich concentration directly inwardly adjacent said case.

12. A wear-resistant article having a steel body provided with an integrally bonded external diffusion case modified by a phase of chromium nitride intermediate the thickness of said case, with an iron-chromium carbiderich concentration directly inwardly adjacent said case.

13. A Weararesistant article having a steel body provided with an integrally bonded external chromized case modified by a phase of chromium nitride intermediate the thickness of said case, with an iron-chromium carbide-rich concentration directly inwardly adjacent said case.

14. A method for the surface hardening treatment of ferrous articles, comprising subjecting the article to a nitriding heat treatment, and then subjecting the article to a chromizing heat treatment to produce a chromized external layer on the article modified by a chromium nitride phase.

15. The method of claim 14, in which the nitriding treatment is carried out in a nitrogen-rich atmosphere having in excess of 1% carbon content, for a ferrous article having, say, about 1.0% carbon content.

16. The method of claim 14, in which the nitriding treatment is carried out in a nitrogen-rich atmosphere having about 1.5% carbon content, for a ferrous article having, say, about 1.0% carbon content.

17. The method of claim 14, in which the nitriding treatment is carried out in a nitrogen-rich atmosphere, for a ferrous article having, say, about 1.0% carbon content, at a temperature of the order of 1535 F.-1550 F. and for a time period inversely related in duration to the percentage of carbon content of the nitriding atmosphere.

18. The method of claim 14, in which the nitriding 7 8 treatment is carried out in a nitrogen-rich atmosphere temperature of the order of 1535 F.1550 F. and for having in excess of 1% carbon content, for a ferrous a time period of the order of 90-100 minutes.

article having, say, about 1.0% carbon content, at-a temperature of the order of 1535 F .1550 F. and for a References Clted by the Exammer time period inversely related in duration to the percent- 5 UNITED STATES PATENTS age of carbon content of the nitriding atmosphere. 2,048,276 7/1936 Marlies et a1. 29195 19. The method of claim 14, in which the nitriding 3,010,856 11/1961 Seelig et a1 117107.2 X

treatment is carried out in a nitrogen-rich atmosphere having in excess of 1% carbon content, for a ferrous DAVID RECK Prlmary Examme" article having, say, about 1.0% carbon content, at a 10 C. N. LOVELL, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,282,746 November 1, 1966 Thaddeus F. Zlotek et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 32 and 60, for "chromallizing", each occurrence, read "Chromallizing" Signed and sealed this 18th day of February 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissioner of Patents

Claims

1. A METHOD FOR THE SURFACE HARDENING TREATMENT OF FERROUS ARTICLES, COMPRISING SUBJECT TO A NITRIDING HEAT TREATMENT, QUENCHING THE ARTICLE AND THEN SUBJECTING THE ARTICLE TO A CHROMIZING HEAT TREATMENT TO