Characterisation of low-pressure carbonitriding with a high residual austenite content by XRD

CETIM has undertaken a study on low-pressure carbonitriding with high residual austenite content to verify the promising results obtained by other teams for mechanical applications where high performance is sought.

Austenite measurements and residual stress analyses were performed on carbonitrided samples treated by conventional carbonitriding, high residual austenite carbonitriding, high residual austenite carbonitriding and tempering, high residual austenite carbonitriding, tempering, and shot peening.

Austenite measurement is performed by energy-dispersive X-ray diffraction. The equipment used is an X-ray diffractometer equipped with a SiLi diode.

The maximum residual austenite contents are obtained at a depth of 0.1 mm. The surface content is lower, probably due to the lower surface nitrogen content:

• On the sample treated by conventional carbonitriding, the maximum content is 20% (30% targeted) with 20% on the surface.
• On the sample treated by high residual austenite carbonitriding, the maximum content is 61% with 45% on the surface.
• On the sample treated by high residual austenite carbonitriding and tempering, the maximum content is 58% with 44% on the surface.
• On the sample treated by high residual austenite carbonitriding, tempering, and shot peening, the maximum content is 53% in the subsurface layer with 28% on the surface.

Tempering causes a transformation of austenite into martensite, but the transformed fraction is small. In contrast, the impact of shot peening is much more significant. The residual austenite content is almost halved on the surface.