A Novel Concept of Hybrid Treatment for High‐Hardenability Steels: Concomitant Hardening and Paraequilibrium Thermochemical Treatment to Produce Interstitially Hardened/Stabilized Austenite Surfaces

Abstract

Steel components' performance requirements are being strongly increased due to the technological evolution of machines. In most of the cases, surface and bulk material requirements are extremely different, so, an optimised component should present different surface and bulk properties. In such a context, the application of surface engineering processes becomes unavoidable and, to achieve the desired performance at acceptable cost, frequently, new materials and/or processing routes are necessary. The novel concept of hybrid treatment proposed here makes possible to obtain components with such characteristics in a single thermal cycle. So it should be a powerful tool helping to overcome the challenges imposed to the metallurgical industry to produce components that present simultaneously high strength and wear and corrosion resistances at lower costs. This process is the subject of a pending patent BR1020180150758 [1] and this communication has the objective of presenting the new hybrid treatment concept, proving its feasibility by means of a case study. The hybrid process, applicable to highhardenability steels, consists in carrying out a thermochemical treatment during the isothermal step of the martempering or austempering hardening treatment (see Figure 1), above the martensite start temperature and so in the metastable austenite field. Thus, this process combines a surface treatment and a substrate bulk treatment, in a same thermal cycle, probably leading to the reduction of the treatment costs as a whole (hardening plus thermochemical treatment). In addition, it enables to produce a hard interstitially hardened/stabilized austenite treated surface, supposedly presenting higher wear and corrosion resistances than the substrate bulk material. In this concept, depending on the steel hardenability and the chosen processing parameters, the bulk material can be either martensitic or bainitic. The stabilized austenite surface can be produced by paraequilibrium nitriding or carburizing thermochemical treatment. The new concept of hybrid treatment is schematically shown in Figure 1. The first step consists in heating a high-hardenability steel component until the austenitizing temperature, waiting for its complete transformation to stable austenite, and this is the "zero" time point in the Figure 1 scheme. In the sequence, the component/part is cooled down to a temperature that enables the paraequilibrium thermochemical treatment to be conducted, in the metastable austenite field, avoiding the martensitic transformation. The paraequilibrium thermochemical treatment step, in the present case nitriding, is successfully performed at temperatures usually below 420 C for austenitic steels. [2-4] As soon as the paraequilibrium thermochemical treatment step is concluded, the component/part can be cooled down to the room temperature. The last cooling step can produce different microstructure in the component bulk (see Figure 1): 1) for the process ending in "A", a martensitic bulk material will be produced;

Kinetics of the UNS S32750 Super Duplex Stainless Steel Low-Temperature Plasma Nitriding

“…. A double layer comprising a double-structure nitrogen-expanded austenite layer was also recently observed in Toscano et al 53 . According to Toscano et al 53 , the occurrence of this phase indicates the formation of nitrogen-expanded austenite layer with double structure, namely an outermost layer with a higher nitrogen content (varying on the 20-26 at.% N range) and an sublayer with lower nitrogen content (varying on the 4-10 at.% N).…”

“…A double layer comprising a double-structure nitrogen-expanded austenite layer was also recently observed in Toscano et al 53 . According to Toscano et al 53 , the occurrence of this phase indicates the formation of nitrogen-expanded austenite layer with double structure, namely an outermost layer with a higher nitrogen content (varying on the 20-26 at.% N range) and an sublayer with lower nitrogen content (varying on the 4-10 at.% N). From Williamson et al 54 , it is stated that the γ' N phase formation is caused by stress-assisted diffusion, but here it could be also related to the just transformed ferrite between the outermost γ N phase layer and the non-chemically altered substrate bulk.…”

Cavitation and strain-induced transformation: the austenite phase behavior in a soft martensitic and an austenitic stainless steel *

“…The two studied austenite kinds were the reversed austenite (γ REV ) of the ASTM CA-6NM martensitic stainless steel (MSS), and the thermallystable austenite (γ EST ) of the AISI 304 austenitic stainless steel (ASS). The former typically occurs in the microstructure of ∼620 °C-tempered CA-6NM MSS substrates at 10-30 vol% range, depending on the steel chemical composition [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The latter occurs in solution-treated 304 ASS substrates [17][18][19].…”

“…It can also be seen that special attention has been given in the last years aiming to determine the effects of nitriding on the CA-6NM steel surface characteristics [8][9][10][11][12][13]. Such studies have shown that high surface hardness can be achieved in this steel due to formation of phases like nitrogen-expanded austenite, γ′-Fe 4 N and ε-Fe 2-3 N and CrN phases, depending on the nitriding temperature and time and the gas mixture used in the treatment.…”

Effects of the Voltage and Pressure on the Carburizing of Martensitic Stainless Steel in Pulsed DC Glow Discharge

“…Concerning the influence of the processing parameters studied here, the literature is rich for austenitic stainless steels (ASSs) plasma nitriding [19][20][21][22][23][24][25][26][27][28][29] , but very poor for MSSs, being mainly focused on treated surface characteristics of different materials, through different processes [30][31][32][33][34][35][36][37][38][39][40] . It is to be noted that almost nothing has been published aiming to study such effects on the MSSs LTPC.…”