Metal Science and tteat Treatment

P'oL 38, Nov. 9

10, 1996

U D C 621.785.5:621.762

E X P E R I E N C E IN C H E M I C A L HEAT T R E A T M E N T OF PARTS M A D E OF P O W D E R MATERIALS FOR V A Z PASSENGER CARS V. T. Koftelev and E. N. Rudenko Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 18 - 2 1 , October, 1996. Every car of the Volzhskii Automobile Plant consists of 1.6 (VAZ-2106) to 4.0 (VAZ-21 l 0) kilograms of parts fabricated from powder materials. By the middle of 1996 the range of these parts (weighing from 1.5 to 600 g) consisted of 150 items. They are fabricated from powders based on iron and copper (119 and 31 items, respectively). Almost half of the iron-base powder parts are subjected to various kinds of surface treatment, namely, carburizing, cyaniding, gas nitriding, and steam oxide treatment. The present paper is devoted to the special features of chemical heat treatment (CHT) of parts produced from iron (steel) powder materials.

In order to create a' carburizing atmosphere in the furnace, a mixture of endogenic gas and methane is fed into it. The composition of the working atmosphere is 3 2 - 38% H 2 , 3 4 . 6 -

Carburizing. This kind of chemical heat treatment is widely used in manufacturing parts from compact steels and is seldom used in powder metallurgy. This arises from the fact that in contrast to cast parts, parts made of powder materials I change size considerably after carburizing and subsequent quenching. Just as in the case of compact steels, parts made of powder materials are subjected to carburizing and quenching in order to increase their hardness. The presence of a denser surface in porous materials increases their strength. For example, the radial compressive strength of the bearing of a ball support increases by 1 2 - 13% after carburizing and quenching. The increase in the strength of powder materials in carburizing and quenching is caused not only by saturation of the surface layers with carbon but also by an increase in the carbon content over the entire volume of the parts due to their porosity. However, this decreases their ductility. O f all the carburizing techniques, gas carburizing, which eliminates the penetration of extraneous materials into the pores o f the part (for example, in carburizing in a solid carburizer), is the most suitable for powder materials. At VAZ parts made o f powder materials are subjected only to gas carburizing. Parts made of powder materials are carburized and quenched in installations shipped to VAZ by the Humbert firm. Before the process the parts are "poured" into mesh containers that are fed into the furnace by a roller conveyer (Fig. 1).

42.0% N 2 , 18 - 2 0 % CO, 2 - 3 % CH 4 , < 0.3% CO 2 , < 0.1% 02 . The concentration of methane in the furnace is controlled by its proportion in the mixture of the endogenic gas and methane. If the concentration of methane exceeds 4.3% (14-to-0.6 proportion of endogenic gas to methane); the surface of the parts is covered with carbon black. 2 Carburizing is conducted at 875 + 5°C and lasts 2.6 - 3 h depending on the dimensions and density of the parts. At the In addition to the proportion of the volumes of the fed-in endogemc gas and methane, the amount of carbon black can be affected by a low carburizing temperature, poor mixing of the two gases, and the presence ofoil on the parts and on the mesh conveyer. Carbon black worsens the appearance of the parts; converting to coke on the surface of the parts, it hampers diffusion of carbon into the metal and worsens the quality ofcarburizing.

The authors consider "powder" parts that have internal pores in their final form, which worsens their quality. An alternative is dense poreless parts produced from powder materials (hot deformation provides 100% density). The latter have better properties than parts produced by the conventional metallurgical method (for example, from high-speed steel) (Ed. note).

Fig. 1. An installation for carburizing and quenching supplied by the Humbert firm.

431 0026-0673/96/0910-0431515.00 © 1997 Plenum PublishingCorporation

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V. 1l".Koftelev and E. N. R u d e n k o

,~'/D, %

HRC 30

7

2

I

0

~

10 15.2 -- &l

15A 15.0 14.9 F

7 r, g/cm3 S

K

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Fig. 2. Variation of the external diameter D, internal diameter Din, and height H of ball-suplmrt bearings in the process of their fabrication from Fe +

5% Cu powder(the verticalsegmentscorrespondto the maximumdimension allowance, the hatched regions reflect the scattering): F) shaping, S) sinte~ ing; K) sizing; C) carbtmzing; H) hardening.

end of the carburizing process the temperature in the furnace decreases to 850°C. After a 1-h hold at this temperature the containers with the parts arrive at the built-in quenching tank filled with MZM-26 oil heated to 50 - 60°C. The total time of the carburizing and quenching process from loading to unloading of the parts is 3.5 - 4.0 h. The quality of carburizing and quenching of parts made of powder materials is monitored by the method of a calibrated file, the radial compressive force that causes failure, and the thickness of the carburized layer. It is known that at some enterprises the thickness of the carburized layer of parts made of powder materials is estimated similarly to parts made of compact steels by a metallographic analysis of specimens. Our experience has shown that this method does not give satisfactory results due to difficulties in determining the boundary of the layer, because carburizing takes place both on the surface of the part and on the surface of the pores in its body. For this reason, at the beginning of the 1970s VAZ turned to a rapid analysis of the carburized layer of parts made of powder materials using blank specimens made of compact steel 20 with a diameter of 15 mm and a length of 40 mm [1]. Such specimens are placed (pairwise) in a certain part of the container together with the powder parts before carburizing and quenching. The determination of the thickness of the layer in the blank specimens by the conventional metailographic method gives highly reproductible results that are well correlated with the quality of the parts. 3 Earlier works in this journal have shown that the parameters of the layer in blank specimens do not coincide with the charactristics of actual parts (see, for example, A. V. Supov, Metalloved. Term. Obrab. Met., No. 5, 8

(19%)). (E& note).

Fig. 3, Dependence of the hardnessHRCand the relative change in the external diameter AD/D in specimens 25 x 15 mm in dimension alter ea~urizing on the initial density y of the Fe + 5% Cu powder material.

Since it is impossible to size parts after earburizing and quenching, the required size allowance for VAZ parts is determined by studying their size variation in carburizing. The knowledge of the regular features of the size variation in parts after CHT makes it possible to establish the range of parts to be shaped by pressing (hot or cold depending on the geometry of the powder) with subsequent carburizing and quenching. The optimized parameters are the sizes of the parts, the admissible size deviations, the composition, the density of the powder material, and the parameters of the technological process. The diagram presented in Fig. 2 has been plotted by statistical processing of the parameters for a sample of 50 parts and shows the expected variation of the main dimensions o f a ball-support bearing made of an Fe + 5% Cu powder for different technological operations, including chemical heat treatment. It follows from the diagram that carburizing and quenching cause an increase in all dimensions of the part. The scattering of the dimensions increases (for example, for the internal diameter it amounts to _+40 lain) and is virtually equal to that in sintering. The diagram shows that sizing has a positive effect on the scattering of the dimensions of a part after carburizing and quenching. The variation of the properties and the dimensions of powder materials after carburizing and quenching is affected substantially by their density. It can be seen from Fig. 3 that the higher the density of powder specimens the more they expand after carburizing and quenching. This occurs because with an increase in the density of powder materials their thermal conductivity increases too, and so does their hardenability. Quenching of a denser powder material results in formation of an elevated amount of martensite.

Experience in Chemical Heat Treatment of Parts Made of Powder Materials

Figure 3 presents the dependence of the hardness 4 of a carburized and quenched powder material on its density. We can see that the function H R C =f(y) has a maximum. The material with a low density has a low hardness, obviously due to the large amount of pores and the low hardenability; the hardness of the material with a higher density is limited by the difficulty of carbon charging of it due to the small number of pores and their closed nature. Cyaniding. Parts made of powder materials are subjected to cyaniding in SNTsA chamber furnaces and in a Humbert through-type furnace in the heat treatment shop of the Demitrovgradsk Plant of Automatic Equipment. The technological parameters of the process, namely, the heating and saturation time and the tempering temperature and time, have been optimized experimentally for each kind of part. These parameters differ slightly. Cyaniding and subsequent heat treatment of the parts are conducted by one of the following variants: (A) cyanJding at 860 - 870°C for 60 - 80 min, quenching in oil, tempering at 180 - 200°C for 40 - 60 min; (B) cyaniding at 860 - 870°C for 90 min, quenching in oil, tempering at 160 - 180°C for 30 - 4 0 min. In both variants the temperature of the parts decreases by 20 - 30°C when they are transported in a container from the furnace to the quenching tank. In each batch the hardness of the parts is monitored by the method of a calibrated file, and the thickness of the saturated layer is monitored by investigating its microhardness and structure (the thickness of the cyanided layer is determined by the distance from the surface of the part to the layer containing equal volume fractions of martensite and troostite). In parts treated by variant A the thickness of the saturated layer h i > 0.2 ram, and the hardness is 40 - 45 H R C ~ . T h e dimensions of the parts increase by 0.07 - 0.15% in cyaniding depending on the composition and density of the material. Parts cyanided by variant B have h t _>0.5 mm and a hardness of 55 - 61 H R C e . T h e dimensions of the parts grow by 0.20 0.30%. On the whole, cyaniding causes a somewhat lower increase in the dimensions of powder parts than carburizing and quenching. In porous materials with a density "/< 6.5 g/cm 3 cyaniding causes virtually through saturation, which decreases their strength. Gas nitriding. VAZ parts made of partially alloyed UIt-raril-2 powder (Ultrapac LE, Distoloy AE) with a composition of Fe + 4% Ni + 1.5% Cu + 0.5% Mo are subjected to gas nitriding in order to increase their wear resistance. These are important parts like the boss and block of the slip clutch of the transmission synchronizer. In order to eliminate through nitriding, which causes an undesirable increase in the dimensions of the parts, the powder material used is quite dense (after sintering and sizing the boss has a density of at 4 The Rockwell hardness of the powder material was measured. In this case

the depth of the indentation in the surface layer affectssubstantially the porosityof the lower-lyinglayec~whichgives lowerhardnessvaluesthan, for example,in measuringthe hardnessby the methodof a graduatedfile.

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least 7.10 g/cm3), and the parts are subjected to the steam oxidizing described below in order to fill the remaining pores with iron oxides. Gas nitriding is conducted by the following regime in installations produced by the Ihelin firm: heating in an exogenic gas to 350°C for 1 h, nitrogen saturation at 570-580°C for 3.5 h in an atmosphere composed o f 50% exogenic gas and 50% ammonia vapor, cooling in the exogenic gas for 0.5 h. The thickness of the carbonitrided layer (8 - 15 ttm) in the nitrided parts and its hardness (> 300 Hit) with a 5-N load on the indentor are monitored metallographieally. The external diameter of parts increases by 0.02 - 0.08% in gas nitriding, which is substantially lower than in carburizing or cyaniding. Steam oxide treatment. This surface treatment5 is a kind of blueing and is used in powder technology as a simple and reliable means for increasing the corrosion resistance, hardness, wear resistance, and strength of powder materials. Steam oxide treatment is conducted is special shaft furnaces (Fig. 4) described in detail in [4]. The process consists of the following operations: heating the furnace to 450 - 600°C, blowing the furnace with steam in order to eliminate the emission o f moisture from the steam duet, feeding the parts into the furnace in heaps or in special mandrels (the inlet and exhaust steam valves are kept open), setting the temperature controller of the furnace at 510°C and a hold at this temperature up to the complete burning of the oil6 (determined by cessation of the smoke emitted from the steam outlet pipe of the furnace), setting the temperature controller at 520 - 540°C, a hold for 1.5- 2.0 h (the valve of the steam outlet pipe is partially closed in order to maintain the steam pressure at a level of at least 100 Pa), switching off the heating of the furnace, cutting the steam inflow at a furnace temperature of 300 - 350°C, and unloading the parts. These parameters of the technological operations are standard and can be changed within a certain range depending on the required degree of oxidizing, the mass of the charged parts, and their composition and density. The degree of oxidizing C is determined from the mass fraction (%) of oxygen absorbed by the part in the steam oxide treatment, i.e., C = m2-ml ×

100,

ml where m I and m2 correspond to the mass of the part before and after the treatment. Calculation by this formula gives a somewhat underestimated value of C, because it does not take into account the amount of oil burnt out in the process. However, in most cases the accuracy of such a determination of C is admissible. 5 In powdermetallurgysteam oxidation is commonlyconsideredas a kind of chemicalheat treatment,because in long-termthermalholds in a steam medium the composition,st~_ctur~,and propertiesof the surfacelayersof porous powdermaterialsarc changed. 6 The oil penetratesthe pores in theirsizing.

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V.T. Koftelev and E. N. Rudenko

monia. Knowing the mass of the part after the oxidizing (m 2 ) and after the reducing annealing (m 3 ) the exact degree of oxidizing can be found by the formula C = m2- m3x 100. m3 As a rule, steam oxide treatment of powder parts with a density 7 = 5.0 - 6.5 g/cm 3 results in a degree of oxidizing of 3 - 6%. Under a steady regime of steam oxidizing the degree of oxidizing is higher the lower the density of the material. CONCLUSION In order to provide the requisite operating properties almost half of the 150 kinds of parts produced in VAZ from powder materials are subjected to different kinds o f chemical heat treatment, namely, carburizing and quenching, cyaniding and quenching, gas nitriding, and steam oxide treatment. All these kinds of CHT have been mastered well and the stably provide high quality for the parts. Fig. 4. A furnace for steamoxidizingof parts made of powder materials.

REFERENCES

1. V. T. Koftelev and L. A. Chemova, Choosing an Optimum In order to increase the accuracy of the determination of the degree of oxidizing, VAZ specialists use a technique that consists in reducing the oxides in parts subjected to steam oxide treatment. The reduction is conducted in sintering furnaces at about 1000°C in an atmosphere of dissociated am-

Method for Rapid Analysis of the Carburizing and Hardening Quality of Cermet Parts. Inf. Report of Kuibyshev TsNTI, Ser. 31, No. 392-72 [in Russian], Kuibyshev (1972). 2. V. T. Koftelev, A. K. Tikhonov, E. N. Rudenko, et al, "Experience in steam oxidizing of powder parts at the Volzhskii Automobile Plant," Poroshkovaya Metallurgiya,No. 9, 90 - 94 (1989).