The good strength - toughness combination of 2.1/4Cr-1Mo Steel is merely in account of its fine grain size (ASTM.NO.8～9) low carbon Martensite lath matrix with very small retained Austenite which is generally less than 3% volum fraction and completely decomposed by tempering below 300 ℃. It means that the "ONe Step Temperembrittlement" of 2.1/4Cr-1Mo steel which is mainly due to retained Austenite decomposition is very weak and can be neglected. There are no significient dropping in low temperature toughness of low tin (0.0031Sn%) 2.1/4Cr-1Mo steel after 1310 hour long term simulated service isothermal ageing in 300～600 ℃, but apparently dropping are shown of the Tin doped (0.024～0.054% Sn) 2.1/4Cr-1Mo steels being treated of the same isothermal ageing in consequence of the tramp element Tin and impurities P and S segregated to grain boundaries, subboundaries, or martensite lath phase boundaries, where their concentrations might be as high as 20～60 times their average bulk contents, which reduce the cohesion force of the matrix lattic. The fractopography of the fine grain specimens the cohesion force of the matrix lattic. The fractopography of the fine grain specimens are chiefly transgranular clevage along the subboundaries or Martensite phase boundaries whith small plastic dimples on their surfaces. The intergranular fracture with apparent cleavages along the grain boundary, however, may be observable only in the coarse grain (ASTM. No. 4～5) matrix steel specimen fractopography. Although "Two Step Temperembrittlement" in 2.1/4Cr-1Mo steel can be largely eliminated by molybedeum addition which may postpone the segregation process of tramp element Tin and impurities P to the grain boundaries through molybdeum concentrated there as more as 3 times than its bulk content in the matrix, such treatment by no means root up the tramp element harmful effects on the after long term isothermal ageing toughness of 2.1/4Cr-1Mo steel.
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