A process for case hardening steel

Description

[0001] This invention relates to a process for case hardening steel and, particularly, to a process for nitriding, carbonitriding, and carburizing of steel parts.

[0002] There have been many prior are proposals for the nitriding, carbonitriding, and carburizing of steel parts. These have taken the form of cyanide salt baths, cyanide/cyanate salt baths, gas carburizing, and so forth. Up until now, each of these prior art processes has been performed independently of the others. For example, the nitriding process has operating conditions and parameters so different from those needed for carburizing that separate processing equipment and chemicals have to be maintained for each process. As a result, the operating conditions for each conventional process are rather inflexible; and can produce only a narrowly defined type of case.

[0003] Additionally, each of these types of processes has certain serious limitations. The cyanide or cyanide/cyanate processes require baths to be maintained with a relatively high concentration of active ingredients, which is both expensive and presents a toxic material waste problem. Gas carburizing and carbonitriding require a high investment for equipment, high energy consumption, and a need for precise atmosphere control. Pack carburizing is very dirty, time consuming, and limited in scope.

[0004] Furthermore, none of these processes are designed to operate in the 600-760°C (1100-1400°F) temperature range. This range could hold great benefits for case hardening of steel parts in certain applications.

[0005] US―A―2 049 806 discloses a process for forming a hardened case on a ferrous article by immersion in a fused salt bath, e.g. of halides and/or carbonates of alkali metal or halides of alkaline earth metals, but does not rely on the conventional cyanide or cyanate compounds. instead, solid non-metallic organic cyanogen compounds e.g. cyanamide, dicyanamide or melan, are added to the bath preferably in small amounts as the work progresses. The best carburising results are stated to be obtained at 700-950°C, but temperatures of 500 to 700°C are also indicated as suitable where a high degree of carburisation is not required. US-A-3 303 063 discloses a salt bath mixture including halides and carbonates of alkali metal and/or alkaline earth metals, which produces cyanate compounds. US-A-2 541 085 describes a similar process, also, however, relying on conventional cyanide in large quantities to perform the case hardening.

[0006] The present invention is based on the problem of providing a process for forming a hardened case on a ferrous metal workpiece by disposing the workpiece in a fused chemical salt bath at elevated temperature, which is efficient and which does not require the bath to be maintained with a high concentration of active ingredients, so as to avoid problems of toxic waste associated therewith, and which at the same time is flexible so as to permit a wide variety of cases to be formed on the workpiece.

[0007] The problem is solved by providing a process in such a salt bath comprising the steps of a) providing a fused chemical salt bath consisting of a first material selected from alkali halides, alkaline earth halides or mixtures thereof, and a second material selected from alkali oxides, alkaline earth oxides, alkali carbonates, alkaline earth carbonates or mixtures thereof, said second material being present in an amount of from about 1 to about 20% by weight of the fused chemical bath; b) maintaining said bath at a temperature of 540°C to 950°C and free of cyanide and cyanate salts; c) suspending the metal workpiece in said bath; d) thereafter adding to said bath a third material or combination of materials selected from urea, dicyanodiamide, any of the pyrolysis/condensation products of urea or dicyanodiamide, or combinations thereof at a rate which will not produce cyanide or cyanate salts, and producing a case on said workpiece from the in situ formed case producing ingredients; e) removing said workpiece with the case thereon from the bath; and f) thereafter maintaining said bath without further addition of said third material prior to the introduction of the next workpiece.

[0008] The single figure is a graphical representation of the optimum rates of addition of urea to the fused bath as a function of temperature and size of the load being treated. The graph is intended as a rough guideline for most low carbon, low alloy steel treatment.

[0009] According to the present invention, a method and composition are provided which allow one to selectively nitride, carbonitride, or carburize a steel workpiece to provide the desired case thereon. The method employs a fused, nonactive chemical salt bath to which is added an organic material which will form certain carbon and nitrogen compounds, said compounds having the ability to add carbon, nitrogen, or both to a steel surface immersed in the bath.

[0010] As used herein, nonactive bath means a bath which itself neither generates nor contains "active" compounds. "Active" compounds are those which, upon giving up carbon and/or nitrogen to the steel to form a case thereon, are transformed into nonactive compounds.

[0011] The characteristics of the case on the steel will depend primarily on the temperature of the bath and the rate at which organic material is presented to the bath, so that either a nitrided, carbonitrided, or carburized case may selectively be obtained. A nitrided case is defined herein to mean a case comprised essentially of iron and nitrogen, usually in the form of iron nitrides. A carburized case is defined to mean a case comprised essentially of iron and carbon, and a carbonitrided case is defined as one which contains iron with significant amounts of both carbon and nitrogen.

[0012] The organic material usually is added only so long as there is work in the bath to be treated. After the treatment is finished, the addition of material may be ceased. The result is that carbon and nitrogen-containing compounds are no longer generated, and residual active compounds are slowly destroyed by thermal decomposition, or by reaction with the walls of a metallic salt pot if such type of pot is used. Hence, the bath when not in use becomes nonactive. Thus, there is no need to maintain a high concenration of toxic materials, such as cyanides and cyanates.

[0013] Turning now more specifically to compositions of the materials involved in this invention, the nonactive chemical salt bath may be any fused alkali halide or alkaline earth halide, or combination of such halides, with from about 1 to about 20 weight percent added thereto of an alkali oxide or alkaline earth oxide or carbonate, or mixtures of such carbonates and oxides. A preferable composition is from 64-68 weight percent calcium chloride, CaCl₂; 30-32 weight percent sodium chloride, NaCI; and 1-5 weight percent calcium carbonate, CaC0₃. The melting point of this composition is about 510°C. (950°F). The organic material for addition to the bath may be urea or dicyanodiamide. It may also be any of the pyrolysis/condensation products of urea or dicyanodiamide, such as melamine, melem, melam, and melon. It may also be any combination of the aforesaid organic materials.

[0014] In operation, the nonactive bath is first brought. to the desired operating temperature, chosen from within the range of about 540°C. (1000°F.0 to about 950°C. (1750°F). The selection of the temperature will be discussed presently. The organic material is then added to the bath, preferably slowly continuously, and at a constant rate. Somewhat complicated reactions now occur, which may be summarized as follows:

[0015] The amount of cyanamide formed is limited by the initial concentration of oxide/carbonate. Thus, if the initial oxide/carbonate amount was 3 weight percent, the maximum amount of cyanamide that can form in the bath is also 3 weight percent. Excess organic material is thermally decomposed and is lost.

[0016] The workpieces to be treated may be immersed in the salt bath either before or after addition of organic material has begun. The pieces should be clean and dry. The pieces are maintained in the bath for virtually any amount of time, depending upon the case thickness required. The general reaction at a steel surface in the bath is as follows:

oxide (0^-2) and/or carbonate (CO₃^-2) ions+carbon (C) and/or nitrogen (N) for diffusion into steel.

[0017] Thus, it can be seen that nonactive oxides and carbonates, consumed in the generation of active cyanmide ions, are regenerated when the cyanamide decomposes at the steel surface, und hence no waste product salts build up in the bath to interfere in the process.

[0018] Agitation of the bath during operation is desirable, in order that active compounds may be evenly dispersed to the entire load. Agitation is usually accomplished by means of either a stirring mechanism, or by bubbling a gas such as air or nitrogen through the bath.

[0019] Repeated analysis of the bath during the process of this invention while the organic material is being added has shown that no alkali and/or alkaline earth metal cyanide and/or alkali and/or alkaline earth metal cyanate salts are products produced, and in fact no cyanides or cyanates are produced. Thus, the bath is free of alkali and/or alkaline earth metal cyanide or alkali and/or alkaline earth metal cyanate salts, and in fact oil cyanides and cyanates before the process, during the process, and after the process is completed.

[0020] The optimum rates of addition of urea for temperatures between 590°C. (1100°F.) and 950°C. (1750°F.) are shown in graphic form in the figure. As can be seen in the figure, the size of the load being treated has been divided somewhat arbitrarily into three categories; light loads (25 square cm of load surface area per Kg of salt); medium loads (65 sq. cm of surface are of load per Kg of salt); and heavy loads (100 sq cm of surface area of load per Kg of salt). The rates of addition for each type of load are graphed as the addition rate in % per hour based on bath weight as a function of temperature and various from about 0.2% to about 4%. Of course, different load surface areas can be extrapolated from those graphed. As noted, the addition rates are based on urea being added, but slight experimentation will determine the optimum rate for other materials.

[0021] When the desired immersion time is up, the workpieces are withdrawn from the bath and then either slow cooled or quenched in an appropriate quenching medium, again depending upon the case characteristics desired. The addition of organic material, having been continued during the immersion of the pieces, may not be ceased. If desired, organic material addition may be terminated before the pieces are withdrawn, to allow the residual active compounds and the thermal diffusion effects to complete the case formation.

[0022] One great advantage inherent in this invention is the variety of possible case types, arising from the flexibility of operating conditions. The type of case formed on the steel is primarily dependent upon the temperature at which the bath is maintained, the type of quench applied, and the composition of the base steel. In general, for low carbon low alloy steels, the case formed at or below 590°C. (1100°F.) is essentially a nitride case, i.e., a case containing essentially iron and nitrogen. The outermost portion of the case usually consists of iron nitrides s Fen, s Fe₃N, and/or y'Fe₄N.

[0023] Below the nitrides, a zone of nitrogen dissolved in alpha iron usually exists. As the treatment temperature rises above 590°C. (1100°F.), the case will contain progressively more carbon and less nitrogen. Hence, the temperature range of about 590°C. (1100°F.) to about 815°C. (1500°F.) is termed the "carbonitriding" range. In general, within these operating temperatures the case is formed in conjunction with a fast quench, such as with water or oil. The case formed is primarily martensitic or bainitic, because addition of nitrogen to the iron at these temperatures has stabilized gamma iron, into which carbon may readily dissolve. Because the core material is still below its critical transformation temperature, the fast quench forms martensitic/bainitic structures only in the region of the case; the core material is essentially unaffected. If desired, a high organic material feed rate will produce a case type consisting of an outer layer of essentially iron nitride, under under exists an austenitic or pearlitic zone.

[0024] From operating temperatures of about 815°C. (1500°F.) to about 950°C. (1750°F.), the case consists of essentially iron and carbon; this is the "carburized" case. Again, this case is usually fast quenched to produce a martensitic or bainitic structure.

[0025] Of course, if there are different alloying elements present, there will be somewhat more complex cases formed, but there will be essentially those general types described, modified by the alloying elements and the modifications of certain of the critical temperatures as is well known in the art. Thus, it will be seen that all of these different case types can be formed in a single fused salt bath using a single organic addition agent which generates active C and N compounds as needed. The cases are varied by merely changing the operating temperature, the quench conditions, and/or the organic feed rate.

Claims

1. A process for forming a hardened case on ferrous metal workpiece comprising the steps of:

(a) providing a fused chemical salt bath consisting of a first material selected from alkali halides, alkaline earth halides or mixtures thereof, and a second material selected from alkali oxides, alkaline earth oxides, alkali carbonates, alkaline earth carbonates or mixtures thereof, said second material being present in an amount of from about 1 to about 20% by weight of the fused chemical bath;

(b) maintaining said bath at a temperature of 540°C to 950°C and free of cyanide and cyanate salts;

(c) suspending a metal workpiece in said bath;

(d) thereafter adding to said bath a third material or combination of materials selected from urea, dicyanodiamide, any of the pyrolysis/condensation products of urea or dicyanodiamide, or combinations thereof at a rate which will not produce cyanide or cyanate salts and producing a case on said workpiece from the situ formed case producing ingredients;

(e) removing said workpiece with the case thereon from the bath; and

(f) thereafter maintaining said bath without further addition of said third material prior to the introduction of the next workpiece.

2. A process according to Claim 1, characterised in that the temperature of the bath is maintained between about 540°C and about 590°C to form an essentially nitride case.

3. A process according to Claim 1, characterised in that the temperature of the bath is maintained between about 590°C and about 815°C to form a carbonitride case.

4. A process according to Claim 1, characterised in that the temperature of the bath is maintained between about 815°C and about 950°C to form an essentially carburized case.

5. A process according to any one of the preceding claims, characterized in that the first material is a mixture of CaC1₂ and NaCl, and wherein said second material is CaC0₃.

6. A process according to Claim 5, characterized in that said first material includes about 64-68 percent by weight CaCI₂, about 30-32 percent by weight NaCl, and wherein said second material includes about 1-5 percent by weight CaC0₃.

7. A process according to any one of the preceding claims, characterised in that the third material is urea.

8. A process according to any one of Claims 1-6, characterised in that the third material is dicyanodiamide.

9. A process according to any one of Claims 1-6, characterised in that the third material is a pyrolysis/condensation product of dicyanodiamide.

10. A process according to Claim 7, characterised in that the rate of addition of urea is from about 0.1 to about 4 percent by weight of the bath per hour of addition time.

11. A process according to Claim 7, characterised in that the rate of addition of urea is controlled as a function of temperature and load size.

Ansprüche

1. Verfahren zum Aufbringen einer Härteschicht auf eisenhaltige Werkstücke, gekennzeichnet durch folgende Verfahrensschritte:

a) Bereiten einer chemischen Salzschmelze bestehend aus einem ersten aus Alkalihalogeniden, erdalkalischen Halogeniden oder Gemischen davon ausgewählten Material und einem zweiten aus Alkalioxiden, erdalkalischen Oxiden, Alkalikarbonaten, erdalkalischen Karbonaten oder Gemischen davon ausgewählten Material, wobei das zweite Material in einer Menge von 1 bis 20 Gewichtsprozenten der Salzschmelze vorhanden ist,

b) Halten der Salzschmelze auf eine Temperatur zwischen 540°C bis 950°C frei von Zyaniden und Zyanatsalzen,

c) Einhängen eines Werkstückes in die Schmelze,

d) anschließendes Hinzufügen in die Schmelze eines dritten Materials oder einer Zusammensetzung von Materialien ausgewählt aus Harnstoffen, Dizyandiamid, jedem Pyrolyse-, Kondensationsprodukt von Harnstoff oder Dizyandiamid oder Zusammensetzungen davon in einem derartigen Verhältnis, daß keine Zyanide oder Zyanatsalze erzeugt werden und Erzeugen einer Schicht auf dem Werkstück aus den in situ gebildeten schichterzeugenden Bestandteilen,

e) Entfernen des mit der Schicht versehenen Werkstückes aus der Schmelze und

f) Beibehalten der Schmelze ohne weiteres Zuführen des dritten Materials vor dem Einhängen des nächsten Werkstückes.

2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Schmelze von einer Temperatur zwischen ungefähr 540°C und 590°C gehalten wird, um hauptsächlich eine Nitrierschicht zu erhalten.

3. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Schmelze auf einer Temperatur zwischen ungefähr 590°C und 815°C gehalten wird, um eine Karbonitrierschicht zu erhalten.

4. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Schmelze auf einer Temperatur von ungefähr 815°C und 950°C gehalten wird, um eine im wesentlichen karburierte Schicht zu erhalten.

5. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß das erste Material ein Gemisch aus CaCI₂ und NaCI und das zweite Material CaC0₃ ist.

6. Verfahren nach Anspruch 5, dadurch gekennzeichnet, daß das ersten Material ungefähr 64 bis 69 Gewichtsprozente CaCl₂, ungefähr 30 bis 32 Gewichtsprozente NaCI und das zweite Material ungefähr 1 bis 5 Gewichtsprozente CaC0₃ aufweisen.

7. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß das dritte Material Harnstoff ist.

8. Verfahren nach einem der Ansprüche 1 bis 6, dadurch gekennzeichnet, daß das dritte Material Dizyandiamid ist.

9. Verfahren nach einem der Ansprüche 1 bis 6, dadurch gekennzeichnet, daß das dritte Material ein Pyrolyse-, Kondensationsprodukt von Dizyandiamid ist.

10. Verfahren nach Anspruch 7, dadurch gekennzeichnet, daß das Verhältnis der Hinzufügung von Harnstoff ungefähr 0,1 bis ungefähr 4 Gewichtsprozente der Schmelze pro Stunde Additionszeit beträgt.

11. Verfahren nach Anspruch 7, dadurch gekennzeichnet, daß das Beimengungsverhältnis von Harnstoff als Funktion der Temperatur und des Gewichts pro Abmessung gesteuert wird.

Revendications

1. Procédé de réalisation d'une couche durcie par cémentation sur des pièces métalliques ferreuses comprenant les étapes de:

(a) Préparation d'un bain de sel chimique fondu constitué d'une première substance choisie parmi les halogénures alcalins, les halogénures alcalino-terreux ou des mélanges de ces derniers et une seconde substance choisie parmi les oxydes alcalins, oxydes alcalino-terreux, carbonates alcalins, carbonates alcalino-terreux ou des mélanges de ces derniers, ladite seconde substance étant présente en une quantité d'environ 1 à 20% en poids du bain chimique fondu;

(b) Maintien dudit bain à une température comprise entre 540°C et 950°C et sans cyanates et cyanures salins;

(c) Suspension d'une pièce métallique dans ledit bain;

(d) Addition ultérieure audit bain d'une troisième substance ou combinaison de substances choisies parmi l'urée, le dicyano- diamine, l'un quelconque des produits de pyrolyse/condensation de l'urée ou du dicyanodiamide, ou des combinaisons de ces derniers à une vitesse qui ne produise pas de cyanates ou cyanures salins et produisant une couche cémentée sur ladite pièce à partir des ingrédients de cémentation formés in situ;

(e) Extraction de ladite pièce cémentée à partir du bain; et

(f) Ensuite maintien dudit bain sans addition supplémentaire de ladite troisième substance avant l'introduction de la pièce suivante.

2. Procédé selon la revendication 1, caractérisé en ce que la température du bain est maintenue entre environ 540°C et environ 590°C pour donner principalement une couche cémentée au nitrure.

3. Procédé selon la revendication 1, caractérisé en ce que la température du bain est maintenue entre environ 590°C et environ 815°C pour donner une couche cémentée au carbonitrure.

4. Procédé selon la revendication 1, caractérisé en ce que la température du bain est maintenue entre environ 815°C et environ 950°C pour donner principalement une couche cémentée au carbure.

5. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que la première substance est une mélange de CaC1₂ et de NaCl, et dans lequel ladite seconde substance est CaC0₃.

6. Procédé selon la revendication 5, caractérisé en ce que ladite première substance comprend environ 64-68% en poids de CaCl₂, environ 30-32% en poids de NaCl, et dans lequel ladite seconde substance comprend environ 1-5% en poids de CaC0₃.

7. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que la troisième substance est l'urée.

8. Procédé selon l'une quelconque des revendications 1 à 6, caractérisé en ce que la troisième substance est le dicyanodiamide.

9. Procédé selon l'une quelconque des revendications 1 à 6, caractérisé en ce que la troisième subtance est un produit de pyrolyse/condensation du dicyanodiamide.

10. Procédé selon la revendication 7, caractérisé en ce que la vitesse d'addition de l'urée est comprise entre environ 0,1 et environ 4% en poids du bain par heure de la durée d'addition.

11. Procédé selon la revendication 7, caractérisé en ce que la vitesse d'addition de l'urée est régulée en fonction de la température et de la taille de la pièce.

Drawing