The Impact of Heating Rate on the Kinetics of the Nitriding Process for 52100 Steel

Abstract

The aim of this study was to determine the impact of the heating rate of steel balls made of AISI 52100 alloy steel on the kinetics and efficiency of the gas nitriding process when carried out using a chemical reactor with precise thermo-gravimetric measurements, which allowed for changes in sample mass during heating and nitriding to be monitored with an accuracy of 50 µg. In the chemical reactor, the examined alloy steel was subjected to a heating process at the selected nitriding temperature of 590 °C. Two heating variants were used: the first variant relied on heating to the nitriding temperature with different rates-1 °C per minute, 2 °C per minute, 5 °C per minute and 10 °C per minute, respectively-whereas the second variant relied on the fast-25 °C per minute-heating of treated specimens to a temperature of 475 °C, at which, the nitrogenous potential of the atmosphere promotes faster nitrogen diffusion deep into the nitrided substrate, followed by reheating up to the nitriding temperature at different rates: 1 °C per minute, 2 °C per minute, 5 °C per minute, and 10 °C per minute, respectively. To evaluate the impact of heating rate kinetics and effectiveness during nitriding on the obtained surface layer quality, we investigated the phase composition, microhardness distribution, and thickness of the obtained diffusion layers. It was found that heating to a temperature of 475 °C in the nitriding process does not significantly affect the average mass gain of a sample. Above this temperature, within the range of nitriding temperatures, the extension of time increases the sample's mass gain. Simultaneously, it was found that the use of a constant heating rate allows for thicker nitrided layers and a greater sample hardness to be obtained. Dual-stage heating, in turn, is more effective in the context of sample mass gain per time unit.

Keywords: gas nitriding; kinetics and efficiency of the process; nitride phases.

Figure 1

T—NP equilibrium configuration (Lehrer’s system) [15].

Figure 2

Diagram of a test bench with a thermocouple (1—shutter, 2—reactor wall, 3—reactor furnace, 4—gas phase sampling point, 5—sample holder with single layer of samples, 6—thermocouple, 7—electronic flowmeters, 8—cooling water (closed circuit), 9—ball valve, 10—gas mixer, 11—process control computer, 12—scrubber (distilled water).

Figure 3

Average sample mass change during process proceeding.

Figure 4

(A) X-ray analysis results for samples a and b; (B) microscopic analysis results and process proceeding data for sample a. (C) Process proceeding characteristics (T—temperature, NP (nitrogenous potential, m—mass gain, NP (e/g’)—boundary potential e/g’ as temperature function) for sample a.

Figure 5

(A) X-ray analysis results for samples c, d, and e; (B) microscopic analysis results and process proceeding data for sample c; (C) process proceeding characteristics (T—temperature, NP—nitrogenous potential, m—mass gain, NP(e/g’)—boundary potential e/g’ as temperature function) for sample c.

Figure 6

(A) X-ray analysis results for samples f, g, and h; (B) microscopic analysis results and process proceeding data for sample g; (C) process proceeding characteristics (T—temperature, NP—nitrogenous potential, m—mass gain, NP(e/g’)—boundary potential e/g’ as temperature function) for sample g.

Figure 7

(A) X-ray analysis results for samples i, j, and k; (B) process proceeding characteristics (T—temperature, NP—nitrogenous potential, m—mass gain, NP(e/g’)—boundary potential e/g’ as temperature function) and (C) microscopic analysis results and process proceeding data for sample i.

Figure 8

Hardness measurement results for different heating rates at temperatures above 475 °C.