Controlled gas nitriding of 40HM and 38HMJ steel grades with the formation of nitrided cases with and without the surface compound layer, composed of iron nitrides

The article describes the results of inve stigations of controlled gas nitriding processes of alloyed structural steel grades 40HM and 38HMJ, used for machine components exposed to corrosion, wear and contact fatigue in service. Examples are given of the process desi gn enabling the formation of nitrided cases on alloyed steels with an iron nitride compound layer at the surface, designated for fatigue applications a nd as substrate for duplex processes. Introduction The article is dedicated to problems connected with the formation of nitrided cases on structural alloyed steel grades 40HM (4140 per AISI) and 38HMJ (Nitralloy 135M) with the presence of a su rface iron nitride layer of MAINTENANCE PROBLEMS 2-2006 44 varying phase composition or without this layer, designated for application for precision components of machines, equi pment and vehicles subjected to corrosion, wear and fatigue in service. The aim of this project was to develop nitriding processes enablin g the formation of three types of nitrided cases: – with an iron nitride layer with a dominant ε phase and a microstructure composed of ε + (ε + γ’ precip.), porous in its external zone, which facilitates its impregnation with a rust inhibitor, – with a compact layer of iron nitrides and with a lesser proportion of the ε phase and a composition of ε + γ’ precip., with an external porous zone limited to a minimum ( ≤ 2.5 μm), – without an iron nitride compound layer. All nitrided cases, taking into account their expected exposure to corrosion, tribological or contact fatigue hazards, should be characterized by high hardness as well as appropriate thickness of the iron nitride layer and its particular zones for corrosion resistance (first two cases), or by an appropriately deep effective case depth, to counteract contact fatigue (t hird type of case). Cases containing the compound nitride layer should also be characterized by a limited thickness of the latter, on account of the narrow dimensional tolerances of the components for which they are designated. They should, moreover, meet the requirements regarding corrosion resistance. It should be emphasized that literature concerning anti-corrosion nitriding of alloyed steels contains substantially less publications than in the case of carbon steels. This is especially true of publica t ons dealing with the formation on alloyed steels of thin, corrosion-resistant iron nitr ide compound layers [1-5]. Similarly, the problem of formation of nitrided cases without compound layers on alloyed steels, despite a definite interest in them for fatigue applications, is limited to only very few mentions, these being mainly concentrated on Nitralloys [6-8]. 1. Materials used For the investigations, two alloyed steels were used, the 40HM and 38HMJ grades, widely used for nitriding. These steels were quenched and tempered prior to nitriding. The heat treatment parameters and hardnesses are given in Table 1. Table 1. Parameters of heat treatment Hardness Steel grade Heat treatment parameters HV10 HRC Q. 860C/ oil T. 490C 395 40 T. 570C 329 33 40 HM T. 600C 327 33 Q. 920C/ oil T. 490C 473 47 38HMJ T. 570C 390 40 2-2006 MAINTENANCE PROBLEMS 45 For the investigations, coupons, sized φ 24 x 4 mm, ground to Ra = 0.32 μm were used. 2. Nitriding equipment Nitriding of the coupons was carried out in an industrial pit-type furnace, model Nx609, manufactured by N itrex, with a retort of φ 600 x 900 mm dimensions and computer process contro l. This furnace is equipped with an ammonia dissociator, as well as a system of fast cooling of the load after the process, as well as with a neutralizer of the effluent atmosphere. Computer process control enables precise contro l of the composition, flow rate and nitriding potential of the atmosphere, as well as of the temperature and time of the cycle. It also enables monitoring a nd full recording of all process parameters during each of its stages, very useful in the analysis and design of nitriding cycles. 3. Nitriding processes Nitriding processes were carried out at emperatures within the range of 470 – 580C and times of 4 – 28 h in atmospheres composed of ammonia or a mixture of ammonia and dissociated a mmonia. Process parameters varied, depending on the type of cases and layers produced. Nitriding with the formation of layers with a porous iron nitride zone were carried out at the higher end of the temperature range (570 – 580 C) for 4 h, in atmospheres composed of ammonia with the cycle ending on a nitriding potential Np = 1.97 – 2.76, which facilitated the rapid formation of nitride compound layers. For the formation of nitrided cases w ith a compact iron nitride layer, lower nitriding temperatures were implemented (560 and 530 C), times used were 4 – 12 h and the atmospheres were composed f ammonia and dissociated ammonia, with a lower nitriding potential of 1.38 – 2.56. For the formation of cases free of th e iron nitride layers, the nitriding temperatures were even lower (530 – 470 C), while times were longest, reaching 28 h, and atmospheres composed f ammonia and dissociated ammonia had very low nitriding potentials (0.23 – 0.38). The implementation of low, nonconventionally used temperatures (470 C) facilitated the retention of a high core hardness ( ≥ 40HRC), advantageous from the point of view of contact fatigue strength. In the first two instances (i.e. cases with iron nitride compound layers at the surface), atmospheres with a fixed nitrid ing potential throughout the cycle or its stage were used. In the third instance (nitrided cases without a compound layer at the surface), the nitriding potential was gradually lowered until it reached a MAINTENANCE PROBLEMS 2-2006 46 very low-end value. During the running of the process, control of the atmosphere was implemented throughout th e cycle, including the heat-up stage. Changes in the nitriding potential during the heating up and at temperature were selected, based on the Lehrer diagram in such a way that they were as close as possible to the boundary values of the potential between the α, γ’ and ε phases on that diagram [9, 10]. They were also selected, depending on the needs, below and above the boundary values. Such potential variations were verified experimentally, choosing the most advant ageous running of the process from the point of view of strengthening the nitr ided case and the kinetics of its growth. 4. Post-nitriding investigations Following the nitriding processes, investigations encompassed the following: – surface hardness (H s), maximum hardness (H max) and core hardness (H core), – hardness traverses across the nitrided case, – microstructure of nitrided cases, – phase composition of the iron nitride compound layers, – compound layer thickness (CLT) and of its porous zone (PZT), – effective hardened case depths (ECD core+50, ECD400, ECD600). Hardness measurements were conducted on a semi-automatic hardness tester manufactured by Zwick. Investigations of microstructure of nitrided cases were carried out on crosssections of the coupons, with the aid of an optical microscope manufactured by Zeiss. The optical microscope was also used for the measurement of compound layer thickness, with its particular zones. The phase composition of iron nitrides was determined with the aid of an X-ray diffractometer, employing CoK α radiation. Effective case depths were determined based on hardness distribution curves. 5. Examples of nitriding processes The chosen examples were three processes which enabled the formation on 40HM and 38HMJ grades of three types of nitrided cases with the most advantageous structural and usable feat ures (case depth and hardness) from the point of view of expected applications. Nitrided cases without th e external iron nitride compound layer, designated for applications exposed to contact fatigue stresses were produced on both grades of steel in a process ca rried out at a temperature of 470 C (Nx616). On the other hand, nitrided cases with a compact iron nitride compound layer (Nx610), or a porous layer (Nx613), de signated for anti-corrosion and tribological applications , were produced in processes carried out at 560 C (compact layers) and 570 C (porous layers). Nitriding times of the successive processes were 28, 12 and 4 h. 2-2006 MAINTENANCE PROBLEMS 47