QuestionWe are having a problem achieving a Charpy “V” notch value of 20 J (°°15 ft-lbs) at -29ºC (-20ºF). We are getting 14-16 J (10-12 ft.-lbs.). Our heat treatment involves preheating the material at 500ºC (932ºF) followed by ramping at 1.1ºC/minute (2ºF/minute) to 950ºC (1750ºF), soaking, then gas pressure quenching at 5 bar (nitrogen) and tempering at 640ºC (1185ºF). What can we do to improve the Charpy value?
AnswerTo begin with, it is important to know that the low side of the austenitizing temperature range, 925-950ºC (1700-1750ºF), is recommended when tempering above 565ºC (1050ºF) since it enhances ductility and impact resistance. So, the austenitizing temperature you have selected for this material is correct. However, oil quenching is preferred as it enhances ductility properties (as well as maximizes corrosion resistance).
The tempering temperature you have selected for this material will need to be adjusted to achieve the Charpy values you are looking for. Your tempering temperature is toward the high end of the acceptable range, namely 205-705ºC (400-1300ºF).
Figure 1 shows the effect of tempering temperature on impact properties at room (20ºC) temperature and should help you achieve your goal. Note the shape of the curve and that Charpy V-notch testing reaches minimum values (for the chemistry shown) when tempering at 520ºC (970ºF). You will want to be aware of this minimum when selecting your tempering temperature. Also, Charpy values at -29ºC (-20ºF) will be lower than the data shown in this graph.