**3.1 The influence of process variables and composition of expanded austenite**

The depth profiles for thermochemically hardened stainless steels typically show a trend of increasing depth with higher temperatures and longer process durations. The very hard layer of nitrogen-expanded austenite exhibits a relatively shallow depth with an abrupt transition to the softer substrate material. The high hardness values associated with nitrided

Low Temperature Thermochemical Treatments

atmosphere for further 4 h.

to the flow of nitriding or carburizing gases.

S ymbol Temp. (<sup>o</sup>

of Austenitic Stainless Steel Without Impairing Its Corrosion Resistance 327

carburizing gases. The fluidized bed furnace, which was manufactured by Quality Heat Technologies Pty Ltd has a working chamber of 100mm diameter x 250mm deep with maximum worksize of 70mm diameter x 150mm high. Before charging the samples, the chamber was heated to the treatment temperature of 450oC with the flow of nitrogen gas at 1.05 m3 per hour. Then the samples were charged to the furnace and the treatment gases were introduced and their flow rates were adjusted to meet the required composition, with the total gas flow rate maintained at 0.62 m3 per hour. Table 2. summarizes the process conditions employed in this work. Four different treatments were conducted, including low temperature nitriding, carburizing, hybrid process, and sequential carburizing-nitriding. The hybrid process involved treating the sample in an atmosphere containing both NH3 (for nitriding) and CH4 (for carburizing) for a total duration of 8 h, whilst the sequential process involved treating the sample in the carburizing atmosphere for 4 h and then in the nitriding

furnace having particulate alumina as fluidized particles which flow inside the chamber due

C ) T ime T emp.(<sup>o</sup>

**Nitriding 8N** <sup>450</sup><sup>o</sup> 85 15 8 No treatment 0 00 No

**C arburis ing 8C** <sup>450</sup><sup>o</sup> 5 95 <sup>8</sup> No treatment 0 00 No

**Nitroc arburis ing 8(C +N)** <sup>450</sup><sup>o</sup> 5 80 15 8 No treatment 0 00 No

Table 2. Treatment conditions and their corresponding layer thicknesses.

C H4 N2 NH3 C H4 N2 NH3

G as (% ) G as (% )

**5N** <sup>450</sup><sup>o</sup> 85 15 5 No treatment 0 00 No

**2N** <sup>450</sup><sup>o</sup> 85 15 2 No treatment 0 00 No

**5C** <sup>450</sup><sup>o</sup> 5 95 <sup>5</sup> No treatment 0 00 No

**2C** <sup>450</sup><sup>o</sup> 5 95 <sup>2</sup> No treatment 0 00 No

**5(C +N)** <sup>450</sup><sup>o</sup> 5 80 15 5 No treatment 0 00 No

**2(C +N)** <sup>450</sup><sup>o</sup> 5 80 15 2 No treatment 0 00 No

**Hybrid 4C -4N** 450<sup>o</sup> 5 95 4 450<sup>o</sup> 85 15 4 5.2

The specimens were heated by electrical resistance heating. Prior to treating, the specimens were soaked in concentrated HCl (2 M) solution for 15 minutes duration with the purpose to remove the native oxide film that commonly forms on austenitic stainless steel and protects the metal matrix from corrosion. This oxide layer is believed to act as a barrier for diffusional nitrogen transport (Rie, 1996). After thermochemical treatments, the specimens were quenched in water. The treated specimen cross sections were first characterized by metallographic examination. To reveal the microstructure, the polished surface was etched

**2C -3N** 450<sup>o</sup> 5 95 2 450<sup>o</sup> 85 15 3 1.6 **1C -1N** 450<sup>o</sup> 5 95 1 450<sup>o</sup> 85 15 1 1.37

F irst S tep S econd S tep

C in a fluidized bed

L ayer T hickness (μm)

treatment 8,35

treatment 5,10

treatment 3,26

treatment 3,92

treatment 1,63

treatment 1,20

treatment 4,00

treatment 2,16

treatment 1,25

C ) T ime

Nitriding, carburizing, and hybrid treatments were performed at 450

layer formation are consistent with the large compressive stresses in the residual stress profiles which were determined by XRD.

Previous investigation which regards to the influence temperature and time of thermochemical treatments using Fluidized bed shows that nitriding at 400°C for times up to 6 h could not produce a continuous nitrided layer on the substrate surface. When temperature was increased to 450°C, a uniform layer was formed after 6 h nitriding and was not effective for shorter treatments due to only a very thin discontinuous layer formation after 3 h nitriding, whilst after nitriding for 6 h a layer about 13 μm thick was formed, which has a bright appearance and is resistant to the etchant used to reveal the microstructure of the substrate. Increasing the temperature to 500°C resulted in the formation of a relatively thick nitrided layer after 3h and 6 h nitriding. The morphology of the nitrided layers formed at this temperature for longer treatment times is different from that formed at 450°C for 6h. Some dark phases were formed in the layers which is similar to those observed for plasma nitrided product and can be attributed to the decomposition of the S phase and the formation of chromium nitrides, which is believed reduce the corrosion (T. Bell, 1999).

The microhardness measurement on the nitrided subtrate showing a function of processing time for the three different temperatures. It can be concluded that no obvious hardening was achieved after nitriding at 400°C. The hardening effect is also insignificant after nitriding at 450°C for 1 h and 3h, and at 500°C for 1h. This corresponds well with the above metallographic examinations that no effective nitriding was achieved under these conditions.

According to these, both structural analysis and hardness measurement indicate that under the fluidized bed nitriding conditions, there exists an incubation time for the initiation of nitriding reactions. Nitriding must be carried out for a duration longer than the incubation time in order to produce an effective nitrided layer. From the experimental results, it is also evident that the incubation time is temperature dependent: increasing nitriding temperature reduces the incubation time.

This incubation time phenomena, which has not been reported for other nitriding processes, such as plasma nitriding, may be related to the nature of the fluidized bed process, where the disruption of the native oxide film on the specimen surface, which is required to effect the nitriding reactions, has to rely on thermal dissociation. The higher the temperature, the faster is the dissociation of the oxide film and thus the shorter the incubation time.
