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Group 2 Steels: Medium Carbon Alloy Steels (0.25 – 0.55 %C ) The mechanical properties of medium carbon alloy steels in the normalized condition are not

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Text of Group 2 Steels: Medium Carbon Alloy Steels (0.25 – 0.55 %C ) The mechanical properties of...

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  • Group 2 Steels: Medium Carbon Alloy Steels (0.25 0.55 %C ) The mechanical properties of medium carbon alloy steels in the normalized condition are not very different from those of plain carbon steels with the same amount and distribution of ferrite and carbide phases. The main reason for adding alloying elements is to delay the pearlite and bainite transformations so that martensite can be formed in relatively thick structures during quenching to increase hardenability.
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  • Group 2 Steels: Medium Carbon Alloy Steels (0.25 0.55 %C ) The alloy elements in solution in the austenite at high temperatures will become partitioned between the ferrite and carbide phases after cooling through the eutectoid transformation. Si and Ni dissolve only in the ferrite and cause solution hardening. Mn, Cr, Mo dissolve in both the ferrite and in the cementite where they substitute for Fe atoms in both the bcc and Fe 3 C crystal structures. This redistribution of these solute atoms among the ferrite + cementite phases slows down the eutectoid reaction so that the TTT curves are displaced towards higher times to the right, thereby increasing hardenability. Recall the higher hardenability effect of Cr next slide.
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  • Effect of Chromium on TTT Curves Note the removal of bainite and increased hardenability to form 100% martensite.
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  • Effect of Alloying Elements on Hardenability The relative effects of alloying elements on hardenability is expressed in terms of multiplying factors to be applied to the ideal critical diameter d 1 of a plain carbon steel with a known grain size. These relationships were used to make our hypothetical steel in lecture 8.
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  • Compositions of Medium Carbon Alloy Steels We will discuss the role of alloying additions to each one of these steel alloys.
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  • Effect of Alloying on the TTT Curves of 0.4 %C Carbon Steels 1.Mn Steel AISI 1340 1.58 %Mn retards the start of the pearlite start curve so that it is just possible to obtain martensite by a fast water quench. The rate of the pearlite reaction is also slowed, ie., more time is required to go from start to finish, making it possible to obtain fine pearlite by an oil quench. 2. Ni Steel AISI 2340 3.5 %Ni has a similar effect to Mn, except that it lowers the eutectoid temperature. A fast water quench will give martensite, while a slow oil quench will give fine pearlite. 3. Cr Steel AISI 5140 0.8 %Cr changes the TTT curve into two C-curves where the high temperature curve refers to pearlite and the curve from 600 o C to 300 o C refers to bainite.
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  • Effect of Alloying on the TTT Curves of 0.4 %C Carbon Steels 4. Mo Steel AISI 4040 0.25 %Mo also splits the TTT curve into two C-curves The pearlite curve is displace to higher times so more bainite can be obtained on water quenching. (see next slide)
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  • Effect of Molybdenum on TTT Curves Note the retardation of ferrite and retension of bainite.
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  • Effect of Alloying on the TTT Curves of 0.4 %C Carbon Steels 5. Ni-Cr-Mo steel AISI 4340 The combination of 1.8 %Ni 0.8 %Cr 0.3 %Mo delays the start of both the ferrite and pearlite transformations so that a distinct bay is formed between the pearlite and bainite C-curves. A rapid water quench will give martensite but 100% bainite will form on a moderate oil quench. (see next slide)
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  • Effect of Ni-Mo and Ni-Cr on TTT Curves 100% Martensite
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  • Quenching of Medium Carbon Steels In contrast to low carbon (
  • Tempering of Medium Carbon Steels Medium plain carbon steels must be tempered after quenching, as the lenticular martensite, which forms at carbon concentrations > 0.4%C, can initiate cracks causing the steel to become brittle. (Low carbon steels form lath martensite, which does not cause brittleness so it is not essential to temper these steels) The reduction in hardness and tensile strength by tempering is an approximately linear function of temperature up to 650 o C. An approximately linear increase in ductility, as indicated by tensile elongation, is also observed with increasing tempering temperature.
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  • Tempering of Medium Carbon Steels A quenched and tempered steel may have the same hardness and strength as a fine grained normalized steel but a tempered steel will always have a superior toughness because the spheroidal Fe 3 C particles developed during tempering do not initiate internal cracks in contrast to the thin Fe 3 C plates in the lamellar pearlite formed during normalizing. The more costly quench and temper treatment is thus warranted when conditions require high toughness. Do you recall heat the treatments of steel normalizing, annealing and spheroidizing? see next slide.
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  • Applications of Medium Carbon Steels Medium plain alloy carbon steels are industrially important, not necessarily because they are stronger when quenched and tempered but, because they also display particular properties such as: 1. They can be quenched relatively slowly in oil rather than water to reduce distortion and cracking 2. They can be made fully martensitic in relatively thick sections and can then be toughened throughout the section 3. The Mn steels are strong even in the normalized condition. 4. The Ni steels are tough even at low temperatures. (contd next slide)
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  • Applications of Medium Carbon Steels Medium plain alloy carbon steels are industrially important because: 5. Cr and Cr-Ni steels are more resistant to corrosion, even though the Cr content is well below the level in stainless steels. 6. The Ni-Cr-Mo steels have the highest strength and toughness with a minimum of alloy additions. 7. The Cr-Mo and Cr-V steels are resistant to softening during tempering so are harder than other grades for a given degree of toughness. They also have a greater high temperature strength. In your welding lecture, you heard that medium carbon steels are difficult to weld with 0.4C being an upper limit.
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  • Mechanical Properties of Medium Carbon Steels
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  • The End Any questions or comments?
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  • Summary of simple heat treatments for hypoeutectic and hypereutectic steels. A 1 is the eutectoid transformation temperature. A 3 is the ferrite proeutectoid temperature. A cm is the cementite proeuctectoid temperature. temper

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