Characterization and kinetics of plasma-paste-borided AISI 316 steel

Abstract

In this work, AISI 316 steel was plasma-paste borided in a gas mixture of 70 % H2 -30 % Ar using a mixture of 30 % SiC + 70 % B2O3 as a boron source. The samples were treated at temperatures of (700, 750 and 800)°C for (3, 5 and 7) h. The morphology of the formed boride layers was examined by light microscope and scanning electron microscope coupled to an EDS analyser. The borides present in the boride layer were identified by means of XRD analysis. The boron-activation energy for the AISI 316 steel was found to be equal to 250.8 kJ mol -1 . This value for the energy was compared to the literature data. A regression model based on a full factorial design was used to estimate the boride layers' thicknesses as a function of the boriding parameters: time and temperature. A comparison was made between the values of the boride layers' thicknesses estimated from the regression model with those given by an empirical relation. In addition, an iso-thickness diagram was plotted to predict the boride-layer thickness as a function of the processing parameters. This iso-thickness diagram can serve as a simple tool to select the optimum values for the boride layers' thicknesses for a practical utilisation in industry for this kind of steel. Keywords: plasma paste boriding, kinetics, borides, transition zone, activation energy, regression model V tem delu je bilo jeklo AISI 316 borirano s plazmo v me{anici plinov 70 % H2 -30 % Ar, z uporabo me{anice (30 % SiC + 70 % B2O3), kot vir bora. Vzorci so bili obdelani pri treh temperaturah (700, 750 in 800)°C v trajanju (3, 5 in 7) h. Morfologija nastale boridne plasti je bila preiskovana s svetlobnim mikroskopom in z vrsti~nim elektronskim mikroskopom, opremljenim z EDS analizatorjem. Boridi v borirani plasti so bili identificirani z rentgensko analizo. Aktivacijska energija bora v jeklu AISI 316 je bila 250,8 kJ mol -1 . Ta vrednost je bila primerjana s podatki iz literature. Za dolo~anje debeline borirane plasti v odvisnosti od parametrov boriranja (~as in temperatura), je bil uporabljen regresijski model, ki temelji na upo{tevanju faktorjev. Izvr{ena je bila primerjava debeline borirane plasti, dolo~ene z regresijskim modelom in primerjava s tisto, ki je bila dolo~ena empiri~no. Za napovedovanje debeline borirane plasti je bil postavljen diagram enake debeline v odvisnosti od procesnih parametrov. Diagram enake debeline je uporaben kot enostavno orodje pri izbiri optimalne debeline borirane plasti, za prakti~no uporabo v industriji za to vrsto jekla. Klju~ne besede: plazma boriranje s pasto, kinetika, boridi, prehodno podro~je, aktivacijska energija, regresijski model

Depth Corrosion Characteristics of Borided Layer Produced on AISI 321 Stainless Steel

“…2, where the FeB layer on the top surface (region 1) is followed by an inner layer (Fe 2 B -region 2) formed on the surface of the boronized specimen. Similar microstructures have been were obtained in boronized stainless steels by other authors [18][19][20].…”

Kinetics of Formation of Fe2B Layers on AISI S1 Steel

“…The calculated boron activation energies ranged between 80.70 and 100.16 kJ mol -1 , depending on the value of current density in the range 0.1 -0.4 A cm -2 . It is noticed that these values are lower than those obtained from other reported works 3,12,[21][22][23][24][25][26][27][28] . It should be attributed to the absence of carbon and nitrogen as interstitial atoms leading to the increase in the boron mobility within the material substrate.…”

“…Therefore, the expression describing the evolution of boron diffusion coefficients in Fe 2 B versus temperature is given by Equation 17in the temperature range 1123-1273 K: (17) where R = 8.314 J mol -1 K -1 and T the absolute temperature in Kelvin. Table 3 shows a comparison between the values of activation energy for boron diffusion in Armco iron and some ferrous alloys (steels and gray cast iron) and the estimated value of activation energy for boron diffusion in AISI S1 steel 3,12,[21][22][23][24][25][26][27][28] .…”

Analysis of the growth kinetics of Fe2B layers by the integral method

“…Initial condition : , with wt.% (1) Boundary conditions: for wt.% (2) for wt.% (3) The boron concentration profile is described by the Second Fick's law as follows: (4) where the boron diffusion coefficient is only dependent on the boriding temperature. It is possible to obtain the expression of boron-concentration profile through the Fe 2 B layer using the Goodman's method also called the heat balance integral method (HBIM) [31].…”

“…However, the Fe 2 B phase is more desirable than a double layer (FeB+Fe 2 B) because high tensile stresses develop in the FeB phase which is harder than Fe 2 B [2]. The boriding process can be carried out by using different processes such as: plasma boriding [3], plasma paste boriding [4,5], gas-boriding [6,7], liquid boriding [8], laser boriding [9] and solid boriding (paste or powder) [10][11][12]. Among all boriding methods, only powder packboriding has been widely employed in the industry since it is simpler and more economic.…”