Heat treatment verification
Why heat treatment verification is important?
Steel gets its properties from basic lattice structure, forming process and heat induced microstructural changes. Successful heat treatment, being diffusion based equilibrium phase transformation or rigorously time and temperature controlled phase transformation, is mandatory for most engineering components in demanding applications.
Heat treatment is placed somewhere middle of the component manufacturing process. Unsuccessful heat treatment makes all the processes before and after heat treatment useless. To avoid rework and scrap, quality of the heat treatment should be verified before e.g. grinding process.
How different heat treatment processes effect on material properties?
Hardening and tempering
Hardenability is a measure of steel's ability to form martensite (or bainite) on quenching. Unit for hardenability is length in depth where a certain hardness can be achieved when surface is cooled. Hardness and hardenability are not to be confused. Hardness is measure of material's ability to resist indentation.
Hardenability depends on carbon equivalent in steel. Alloying reduces the demands on cooling rate for formation of martensite. As-quenched martensite generally has severe tensile residual stresses and is hard and brittle. Martensite can form in laths, thin plates or lenticular shape. Tempering relieves residual stresses and makes martensite morphology more favorable for mechanical applications. Reduced harness is trade-off for increased ductility.
Nitriding and carburizing
Increased hardness is improved by introducing interstitial solute atoms in steel structure in atmosphere rich in carbon or nitrogen at elevated temperature and pressure. Acquired hardness and thickness of hardened layer are controlled properties. Residual stresses are introduced during changes in lattice dimensions of the hardened layer.
Annealing and stress-relieving heat treatments
Annealing processes are high temperature applications to relieve stresses, normalize microstructure and grain size after forming, controlling grain size via recrystallisation, reducing segregation, and homogenization of properties.
Residual stress state and texture are reduced by keeping component in elevated temperatures thus allowing diffusion based processes to take place and relieve work applied to microstructure in forms of dislocations, stacking faults, twinning and grain boundaries. Hardness is reduced by releasing the amount stresses worked into the structure.
Grain size control
Engineering materials are seldom single crystals and grain size has significant effect on strength and ductility of materials. Grain size grows and recrystallisation occurs in temperatures higher than stress relieving.
Common heat treatment problems:
- Insufficient hardening
- Tempering issues
- Cooling issues
Heat treatment verification methods
X-ray diffraction method enables measuring absolute residual stress value and retained austenite content. Destructive hardness depth profiles can be measured with X-ray diffraction method.
X-rays have high energy and short wavelength when compared to visible light making them ideal for probing the interplanar distances in crystalline materials.
Hardness and stress directly affect the intensity of the Barkhausen noise signal. With Barkhausen noise method it is effortless to verify success of heat treatment process and separate corrcetly heated parts from non-heated.
Barkhausen noise analysis is a non-destructive method involving the measurement of a noise like signal induced in a ferromagnetic material by an applied magnetic field.
With Stresstech products heat treatment verification and retained austenite measurement can be done effortlessly in production lines, laboratories and even on field.