Transcript Medium Carbon Alloy Steels

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 Hardenability
The 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 ferritepearlite 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 Steels
Some 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?
A1 is the eutectoid transformation temperature.
A3 is the ferrite proeutectoid temperature.
Acm is the cementite proeuctectoid temperature.
temper
temper
Summary of simple heat treatments for hypoeutectic
and hypereutectic steels.