group 2 steels: medium carbon alloy steels (0.25 – 0.55 %c ) the mechanical properties of medium...

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M ech 473 Lectures ProfessorR odney H erring

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Mech 473 Lectures

Professor Rodney Herring

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.

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 Fe3C 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.

Effect of Chromium on TTT Curves

Note the removal of bainite and increased hardenability to form 100% martensite.

Effect of Alloying Elements on HardenabilityThe relative effects of alloying elements on hardenability is expressed in terms

of multiplying factors to be applied to the ideal critical diameter d1 of a plain carbon steel with a known grain size.

These relationships were used to make our hypothetical steel in lecture 8.

Compositions of Medium Carbon Alloy Steels

We will discuss the role of alloying additions to each one of these steel alloys.

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 oC to 300 oC refers to bainite.

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)

Effect of Molybdenum on TTT Curves

Note the retardation of ferrite and retension of bainite.

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)

Effect of Ni-Mo and Ni-Cr on TTT Curves

100% Martensite

Quenching of Medium Carbon Steels

In contrast to low carbon (<0.2 %C) steels, which are extensively used in the hot-rolled condition with little or no alloy additions, medium carbon steels having 0.25 – 0.55 %C are usually heat treated and alloyed to obtain optimum properties.

“All of these steels form hard, strong martensite on water quenching.”

In many cases, martensite is not desired throughout the whole component.

A relatively thick section, which is water quenched will give martensite at the surface for wear resistance, with a ferrite-pearlite core for toughness.

Quenching of Medium Carbon Steels

Surface martensite may also be obtained by heating only the surface using high frequency induction or intense local flames (case hardening -see next slide).

Only part of an assembly may be heated, eg., the bearing sections of a crankshaft, to give hard wearing surfaces where needed.

The whole assembly may be heated but only part may be water quenched, eg., the nose section of pliers, and then the entire assembly can be oil quenched to give a fine tough pearlite structure in the handles.

Case Hardening Stainless SteelsSome special cases of heat treatments or chemical treatments can be used to

preferentially strengthen/harden the surfaces of formed steel parts.

These involve:

• Case hardening

• Nitriding

• Carburizing

Surface hardening by localized heating in a) and the formation of a layer of martensite layer at the surface by heating above the A1 temperature and then quenching.

Carburizing of a low-carbon steel to produce a high-carbon, wear resistant surface.

Quenching of Medium Carbon Steels

In all cases, the maximum hardness is determined by the carbon content of the martensite, which is usually taken to be the carbon content of the steel, on the assumption that all prior Fe3C is taken into solution during the austenitizing treatment before the quench.

Carbides containing Cr and Mo are difficult to dissolve in austenite. If austenitization is incomplete, the full carbon concentration of the martensite will not be obtained, resulting in a lower hardness.

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 oC.

An approximately linear increase in ductility, as indicated by tensile elongation, is also observed with increasing tempering temperature.

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 Fe3C particles developed during tempering do not initiate internal cracks in contrast to the thin Fe3C 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.

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.

(cont’d next slide)

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.

Mechanical Properties of Medium Carbon Steels

The End

Any questions or comments?

Summary of simple heat treatments for hypoeutectic and hypereutectic steels.

A1 is the eutectoid transformation temperature.A3 is the ferrite proeutectoid temperature.Acm is the cementite proeuctectoid temperature.

temper temper