Which of the following powders should be fed for effective oxy-fuel cutting of stainless steel

Which pair among the following solid state welding processes uses heat from an external source
$$P$$- Diffusion welding
$$Q$$- Friction welding
$$R$$- Ultrasonic welding
$$S$$- Forge welding

During machining, the wear land $$(h)$$ has been plotted against machining time $$(T)$$ as given in the following figure.

For a critical wear land of $$1.8mm,$$ the cutting tool life (in $$min$$) is

Diamond cutting tools are not recommended for machining of ferrous metals to