ABSTRACT In terms of reducing the gear noise of automatic transmission, improvement of heat treatment distortion of the annulus gear is very important, because annulus gear is very sensitive heat treatment due to thin walled ring-like shape. Nitriding is very effective method to meet the both requirements for heat treatment distortion and durability of the annulus gear, as compared with conventional carburizing. However, conventional nitriding has problems to be applied for annulus gear, such as brittleness of compound layer and low adhesion strength between compound layer and matrix. In this research, we developed the high toughness nitriding and greatly improved the problems as mentioned above, by controlling gas pressure and temperature. INTRODUCTION Automatic transmission generally uses 2 ∼4 planetary gears to fulfill change of speed and power transmission. Planetary gears are composed of sungear, pinion gear, and annulus gear. The annulus gear is a thin ring type in shape and weak to distortion on heat treatment. ( Fig. 1) Also annulus gear is an internal gear which makes it difficult for grinding after heat treatment. Therefore, method for minimizing heat distortion is necessary. Hence, our company developed low temperature vacuum carburizing method which conducts heat treatment at lower temperature than existing carburizing heat treatment1) but since the nature of carburizing accompanies phase transformation, fundamental improvement is impossible. Nitriding is heat treatment in 500∼ 600°C section that is the temperature under A1transformation point which is very advantageous to reduce heat distortion. Fig 1. Annulus gear However, general nitriding has low pitting resistance comparing with carburization due to the shallow case depth. Also, Carburization hardens with Martensite up to core, improving strength, which nitriding has low hardness and strength since the core keeps material's structure. Also forming a very hard compound layer at the surface is advantageous at durability of abrasion but when applied to gear, compound layer are damaged and peeled off due to the brittleness. That makes difficult to be applied. ( Fig. 2). Fig 2. Damage due to the brittleness in gas soft nitriding In order to apply nitriding to gear, the study tried to acquire durability by improving toughness and adhesion strength by controlling phase of compound layer. Also to increase gear strength, quenching and tempering are applied to acquire higher strength than normalizing. A Development of the High-toughness Nitriding to Reduce Heat Treatment Distortion of AT Annulus Gear2013-01-1769 Published 04/08/2013 Jae Hong Park, Bong Lae Jo, Chang Won Kang, Eui Jun Kim and Hyung Oh Ban Hyundai Motor Company Copyright © 2013 SAE International doi:10.4271/2013-01-1769Downloaded from SAE International by University of Edinburgh, Saturday, August 25, 2018FIRST HEADING OF BODY 1. Principle of Nitriding Nitriding is a method to harden the surface using nitrogen gas under the low temperature of 500∼600°C. Nitriding can be categorized to gas nitriding, salt bath nitriding, gas soft nitriding, plasma nitriding and low pressure nitriding etc. Closely looking into nitriding, when ammonia gas is inert to furnace, nitrogen diffused into the metal after N2 and H2 are formed forms compound such as Fe(2-3)N, Fe4N, etc. Precipitates such as CrN, AlN, VN, CN, etc inside metal harden surface with lattice distortion2) (Formula 1, Fig 3). Compound layer is hard and durable enough to be applied in a wide arrange of automotive parts. So it is much used in automotive parts which demand durability of abrasion. However, it has disadvantage of high brittleness when given an impact to compound layer. (1) Fig 3. Principle of nitriding 2. Property by Controlling of Compound Layer To reduce the brittleness of compound layer, there is a need for research on the characteristics of the compound layer. Nitriding compound layer can be divided into two: ε phase and γ′ phase. Ch