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Carbonitriding and Hard Shot Peening for High-Strength Gears Yoshihisa Miwa, Masayuki Suzawa, Yukio Arimi, Yoshihiko Kojima, and Katsunori Nishimura Mazda Motor Corp

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Carbonitriding and Hard Shot Peening for High-Strength Gears Yoshihisa Miwa, Masayuki Suzawa, Yukio Arimi, Yoshihiko Kojima, and Katsunori Nishimura Mazda Motor Corp Carbonitriding and Hard Shot Peening for High-Strength Gears Yoshihisa Miwa, Masayuki Suzawa, Yukio Arimi, Yoshihiko Kojima, and Katsunori Nishimura Mazda Motor Corp. ABSTRACT CUE HARONESS HARONESS CUE OEtrM A new process for manufacturing high- OISTRIBUTION --E CORE HARONESS strength gears has been developed to meet the requirement of automobile transmission minia- r turization. The points of the process are to GRAIN SIZE increase the shot peening intensity and to MICROSTRUCTURE perform optimal control of the initial (before FATIGUE CARBIOES shot peening) microstructure by heat treatment STREMGTH MON-MARTEMSITIC corresponding with the peening intensity in OEFEtTS -r SURFACEUOU-METALLIC LAYER order to obtain higher residual compressive IuCLUSlONS stress. INTERGRANULAR i TOUGHMESS The new process, named Carbonitriding and Hard Shot Peening in Mazda, brings a much RESlOUAL COYIRESIVE STRESS higher fatigue strength than the one obtained by the conventional carburizing and shot peening process. Fig. 1 METALLURGICAL FACTORS AFFECTING ON THE FATIGUE STRENGTH OF GEARS 1. Introduction A higher drivability, stability in therefore, it is necessary to set a suitable driving, and fuel efficiency are required for i~ardnessdistribution by heat treatment using automobiles and the new systems to meet these clean material. However, in order to augment requirements have been developed. This can the fatigue strength furthermore, it is nec- be seen, for example, in multi-valve system, essary to make use of residual compressive super charging in engines, 4 WD or 4 WS, etc. stress more positively. In addition, the height of engine hood has For enlargement of the residual compressive been lowered in body designing for reducing stress, it is most effective to increase the air resistance. shot peening intensity. At the same time, it Nith such a background as those, it is should be necessary to adjust the initial desired for the transmission system to trans- (before shot peening) retained austenite mit higher engine output in a smaller size in content because the formation of the residual order to enhance the degree of freedom in compressive stress by shot peening is con- designing a power-train system or a suspension ceivably related to the phenomenon of strain- system within a restricted space. induced transformation of retained austenite It is one of the important subjects for the into martensite. material engineers, therefore, to augment the However, there is a problem in increasing fatigue strength of the transmission gears so the peening intensity. That is surface rough- as to meet these requirements. ening which occurs in case of increasing the peening intensity. The surface roughening is made conspicuous due to existence of the soft 2. Concepts of Research and Development structure called non-martensitic surface layer The fatigue strength of the gears is which is inevitably formed in the conventional affected by such factors as shown in Fig. 1. carburized surface of steel. In order to. In order to obtain a high .fatigue strength, prevent the surface roughening, therefore, it 0148-71 9118810229-0666$02.50 Copyright 1988 Society of Automotive Engineers, Inc. is necessary to reduce,.thenon-martensitic the one in carburizing, is formed as well in surface layer. The reduction of the non- the usual carbonitrided surface layer of martensitic surface layer simultaneously serves chromium-alloyed steel. The growth of this to mitigate stress concentration in the surface non-martensitic structure is the technical of steel, as well. difficulty in carbonitriding chromium-alloyed From the above matters, the following steel aiming at improving the fatigue strength. concepts of strengthening gears were introduced: As shown in Fig. 2, this non-martensitic 1) Reducing the non-martensitic surface layer structure is characterized by that the range to mitigate the stress concentration in the of the structure existing along the grain surface of steel and to prevent the surface boundary is deeper than the case of carburizing. roughening when the peening intensity is It is inferable from this fact that the growth increased. mechanism of the non-martensitic structure in 2) Gaining the optimal control of the initial the carbonitrided chromium-alloyed steel should (before shot peening) retained austenite be different from internal oxidation of the content to maximize the residual compressive alloyed elements in the case of carburizing. stress caused by shot peening. 5) Increasing the peening intensity to obtain greater residual compressive stress on con- USUAL dition that the above-mentioned two items CARBONITRIDING are realized. 5. Solution of Technical Difficulty The non-martensitic surface layer in con- ventional carburizing is caused by deterioration of hardenability of the surface layer due to f internal oxidation of the alloyed elements.1) In order to reduce the non-martensitic surface - layer, therefore, it is conceivable to adopt such a method as to accelerate the cooling rate in quenching or to compensate the hardenability of the surface layer deteriorated due to in- b ternal oxidation of the alloyed elements by 3 means of adding other elements. In the above two methods, the former is not realistic because of the fault that the thexmic distortion of the components becomes great. On the contrary, it has been known that nitrogen is an effective element in case of the latter.2) In addition, nitrogen has .an action to stabilize the austenite by lowering the ?IS point of steel.3) From the above consideration, it is con- - ceivably that carbonitriding is hopeful as a Fig 2 NON-MARTENSITIC STRUCTURE OF CAR- technical means to realize the aforesaid BURIZED, USUAL CARBONITRIDED SURFACE development concepts 1) and 2). However, OF CHROMIUM-ALLOYED STEEL carbonitriding has been so far applied widely to carbon steel which does not contain alloyed elements such as chromium, etc., but there are 3.2 Growth mechanism of non-martensitic not so many cases of being applied to alloyed structure of carbonitrided surface layer steel which is used for gears. This is con- of chromium-alloyed steel ceivably due to the reason that the renovation By various experiments and analyses, an effect of hardenability by nitrogen permeation information on the growth mechanism of the is not sufficient on alloyed steels. Neverthe- above-mentioned grain-boundary non-martensitic less. the details of the reaqnn and imnrnvinu -+-..,.+,.-.-. I....- L..--. -L--I--J -- L.-7 7 caused by the growth of chromium nitride is This conception is shown in Fig. 5. A shown in Fig. 4. It is indicated from Fig. 4 new carbonitriding process, shown in Fig. 6, that the distribution of soluble chromium and has been developed based on this conception. that of permeated nitrogen correspond with The process is characterized by that a small each other. amount of NH3 is added within an extremely short time to permeate nitrogen into the ex- tremely shallow surface layer only. f I I EXTENT OF CHROMIUM OXlOATlON OlSTANCE FROM SURFACE mm Fig. 3 STATE OF SOLUBLE CHROMIUM REDUC- TION IN THE CARBONITRIDED SURFACE OF CHROMIUM-ALLOYED STEEL (SCR420) DISTANCE FROM SURFACE Fig. 5 CONCEPTION OF NEW CARBONITRIDING TO MINIMIZE THE GROWTH OF CHROMIUM NITRIDE 0 0' I 840~~x90MIN 1 7 NHJRX = 2.5% , CARBURIZING " rO\*I0\ I 0. I 930aC 0 0.1 0.2 0.3 center \ 1 USUAL DISTANCE FROM SURFACE mm CARBONITRIOINC Fig. 4 DISTRIBUTION OF SOLUBLE CHROMIUM j--J--J CONTENT AND PERMEATED NITROGEN IN THE CARBONITRIDED SURFACE OF NH, 2.5% CHROMIUM-ALLOYED STEEL (SCR420) NEW CARBONlTRlOlNC 3. New carbonitriding process for chromium alloyed steel As aforesaid, the amount of solved chro- mium is very small in carburized surface layer due to internal oxictation. By means of per- meatmg nltrogen into this extremely shallow surface layer only, the growth of chromium nitrlde can be minimized. Fig. 6 NEW CARBONITRIDING PROCESS 4. Microstructure Cont,rol by New Carbonitriding surface layer obtained by the new carbonitrid- Process ing process. The depth of non-martensitic surface layer is extremely small compared with 4.1 Reduction of non-martensitic layer of the case of conventional carburizing or car- carburized surface bonitriding by usual process, and is several Fig. 7 shows the microstructure of the um or under. Fiq. 7 MICROSTRUCTURE OF THE SURFACE LAYER OBTAINED BY NEW CARBONITRIDING PROCESS 4.2 Control of retained austenite content 5. Enlargement of Residual Compressive Stress By means of regulating carbon potential The tests were conducted to optimize the (C.P. ) and nitrogen potential (N.P.) based on shot peeing condition and the initial micro- the new carbonitriding process shown in Fig. 6, structure (retained austenite content) aiming the retained austenite content in the surface at obtaining greater residual compressive layer can be controlled. The range of the stress. The test results are shown hereunder. control is 20 - SO%, which covers more amount of retained austenite content than the case of the carburizing process only as shown in Fig. 8. 5.1 Increasing the peening intensity The main factors to effect the peening intensity are shot hardness and shot velocity. The effects of them on the residual compressive stress are shown in Figs. 9 and 10 respectively. The harder the shot hardness is, the greater the residual compressive stress is made. However, when shot hardness is HRCSS or over, X ,- its working effect seems to be saturated. In > addition, the higher the shot velocity is, the C IY greater the residual compressive stress is made. 2I- However, judging from the present installed U0 capacity of shot peening, it is difficult to W C- obtain a shot velocity of over 100 m/sec. Z Y The new shot peening method shown in Table I- V)a 1 has been developed based on the test results 4 0 as above-mentioned. The characteristics of the WI method are that the shot hardness is greater 4 +Y and the shot velocity is higher than those of a the conventional method.
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