8.4·Alloyed Steels 149 tion is seldom entirely completed even at very low temperatures. This results in some retained austenite,as indicated in Figure 8.6.Retained austenite can be of concern after tempering in that it may lead to some brittleness due to its transformation to martensite upon tempering and cooling to room temperature. 8.4·Alloyed Steels Alloying elements,such as Mn,Si,Ni,Cu,Mo,and V,are often added to steel in various quantities (often well under 1%)in or- der to favorably alter its properties.These additional constituents generally shift the nose in a TTT diagram to longer times.As a consequence,no pearlite or bainite is inadvertently formed upon quenching,and the martensitic transformation can be brought to completion even in large work pieces despite the fact that the cooling rate might have been relatively slow.This feature is re- ferred to as hardenability and expresses the ease with which martensite is formed upon quenching. A second effect that alloying elements provide is a shift of the eutectoid composition to lower carbon concentrations.One mass of molybdenum,for example,reduces the eutectoid composi- tion of plain carbon steel from 0.77 to 0.4%C.This has some in- fluence on the primary microconstituent which is formed upon cooling.Specifically,a reduction of the eutectoid composition might lead to primary cementite instead of primary ferrite in a given plain-carbon-steel. Third,some constituents(such as Mn and Ni)considerably de- crease the eutectoid temperature,TE,and the temperatures at which ferrite and cementite are first formed (Figure 8.1).For ex- ample,5%Ni decreases Te of plain carbon steel by about 70C. Other elements,such as Cr,W,and Mo,increase the eutectoid temperature instead.These changes have to be considered when austenitizing treatments are conducted.Fourth,the martensitic start and finish temperatures are reduced by alloying elements. Moreover,the entire TTT diagram might undergo some varia- tions.Fifth,the time needed for tempering is generally dimin- ished by alloying. Stainless Steel Sixth,and nearly most importantly,appreciable additions of chromium to iron (at least 12%)yield corrosion-resistant steels called stainless steels.They derive this property from a protective layer of chromium oxide which forms on the free surface.How- ever,with rising Cr concentrations the amount of austenite de- creases in either binary Fe-Cr or some chromium-containing iron-carbon steels which cause the ferrite to be the dominant mi-8.4 • Alloyed Steels 149 tion is seldom entirely completed even at very low temperatures. This results in some retained austenite, as indicated in Figure 8.6. Retained austenite can be of concern after tempering in that it may lead to some brittleness due to its transformation to martensite upon tempering and cooling to room temperature. Alloying elements, such as Mn, Si, Ni, Cu, Mo, and V, are often added to steel in various quantities (often well under 1%) in or￾der to favorably alter its properties. These additional constituents generally shift the nose in a TTT diagram to longer times. As a consequence, no pearlite or bainite is inadvertently formed upon quenching, and the martensitic transformation can be brought to completion even in large work pieces despite the fact that the cooling rate might have been relatively slow. This feature is re￾ferred to as hardenability and expresses the ease with which martensite is formed upon quenching. A second effect that alloying elements provide is a shift of the eutectoid composition to lower carbon concentrations. One mass % of molybdenum, for example, reduces the eutectoid composi￾tion of plain carbon steel from 0.77 to 0.4% C. This has some in￾fluence on the primary microconstituent which is formed upon cooling. Specifically, a reduction of the eutectoid composition might lead to primary cementite instead of primary ferrite in a given plain-carbon–steel. Third, some constituents (such as Mn and Ni) considerably de￾crease the eutectoid temperature, TE, and the temperatures at which ferrite and cementite are first formed (Figure 8.1). For ex￾ample, 5% Ni decreases TE of plain carbon steel by about 70°C. Other elements, such as Cr, W, and Mo, increase the eutectoid temperature instead. These changes have to be considered when austenitizing treatments are conducted. Fourth, the martensitic start and finish temperatures are reduced by alloying elements. Moreover, the entire TTT diagram might undergo some varia￾tions. Fifth, the time needed for tempering is generally dimin￾ished by alloying. Sixth, and nearly most importantly, appreciable additions of chromium to iron (at least 12%) yield corrosion-resistant steels called stainless steels. They derive this property from a protective layer of chromium oxide which forms on the free surface. How￾ever, with rising Cr concentrations the amount of austenite de￾creases in either binary Fe–Cr or some chromium-containing iron–carbon steels which cause the ferrite to be the dominant mi￾Stainless Steel 8.4 • Alloyed Steels