Valves are among the most important components controlling a car’s engine. They regulate both the intake of air for the combustion process and the discharge of the exhaust gases generated in the combustion chamber. Exhaust valves, run much hotter than intake valves (which are cooled by the incoming fuel: air mixture) and must withstand the corrosive effects of hot exhaust gases and the weakening effects of high temperatures. Consequently, a premium valve material is an absolute must on the exhaust side

In addition to the use of specially developed materials, the exhaust valve is also nitrided to produce a layer typically 1mm in thickness. This is performed by heating valves in an atmosphere of Ar containing 20% N2 (by weight). Assume that this concentration is at the surface and is kept constant.

The goal is to produce a phase fraction of Fe4N of 10% (by mass) at a depth of 1mm.

Determine a suitable nitriding time and temperature. As a result of phase changes that will occur at temperatures above 592°C, you are limited to temperatures at or below 590°C. Remember that long cycle times are expensive and short cycle times (

 
  

Temperature C 1000 900- 800 700 600 500 400 912C 300 0 Magnetic Trans. A770C -(aFe) 0.10 (yFe) Fe 10 2 2.8 650C Atomic Percent Nitrogen 4.5 6.7 592C 5.7%,680C Fe N 480C Magnetic Transformation 5.7 20 6 508C 8 W Fe-N Phase Diagram 30 FeN Weight Percent Nitrogen 10 12 40 14