EFFECT OF ULTRASONIC VIBRATIONS ON AGE HARDENING BEHAVIOR OF 17-4 PH (MARTENSITIC) STAINLESS-STEELS

The influence of ultrasonic vibrations (for 5 min at 150-degrees-C) on the ageing behavior of 17-4 PH stainless steels was studied in the temperature range of 450-550-degrees-C. It was noticed generally that two precipitates were formed one after the other, the first at around 60 min and the other at around 120 min of ageing. At the high temperature end of ageing (i.e. 550-degrees-C) the first peak shows up faster (about 30 min) while at the low temperture end (i.e. 450-degrees-C) it shows up in about 60 min and even then is only just discernible as a shoulder in an otherwise continuously rising curve. The first precipitate quickly loses its coherency with the parent matrix as evidenced by the sharp overageing which is markedly enhanced by the ultrasonic vibrations. Ultrasonic insonation always increases the hardness of the first peak. However it also emphasizes the subsequent overageing which detracts from the increase in hardness available from the second precipitate. Hence (except for 550-degrees-C), precipitation at longer ageing times (i.e. 4 h) produces a lower hardness following ultrasonic pretreatment. In contrast with this, the strengths (both tensile and impact) are increased both for short time (1 h) and long term (4 h) ageing through ultrasonic treatment. The impact strength shows a minimum in all cases for 90 min ageing at 480-degrees-C and the fracture is brittle. The first precipitate is probably elemental copper (or a Cu-rich phase) while the second is possibly a chromium-rich bcc phase or a carbide or carbonitride phase. The first becomes quickly incoherent with the matrix and leads to overageing while the second is distinctly more coherent with the matrix and softens less, at least in the lower range of ageing temperatures. Different activation energies for kinetics of the two precipitates would account for the ageing profiles noticed. Dislocation density values calculated from lattice microstrain agree with estimates obtained from TEM studies.