The term high-strength steel is often applied to all
steels other than mild low-carbon steels. The steels which
have yield strength over 560 MPa are sometimes called the
ultra-high-strength steels or super alloys.
The groups of steels that fall into this category are:
- Medium-carbon low-alloy hardenable steels
- Medium-alloy hardenable or tool and die steels
- High-alloy hardenable steels
- High-nickel maraging steels
- Martensitic stainless steels
- Semi austenitic precipitation-hardenable stainless steels
Medium-Carbon Low-Alloy Hardenable Steels
The best-known steels in this class are AISI 4130 and
AISI 4140 steels. Also in this class are the
higher-strength AISI 4340 steel and the AMS 6434
steel. These steels obtain their high strength by heat
treatment to a full martensitic microstructure, which is
tempered to improve ductility and toughness.
Tempering temperatures greatly affect the strength levels of
these steels. The carbon is in the medium range and as low as
possible but sufficient to give the required strength.
Impurities are kept to an absolute minimum because of
high-quality melting and refining methods.
These steels are available as sheets, bars, tubing, and light
plate. The steels in this group can be mechanically cut or
flame cut. However, when they are flame cut they must be
preheated to 316°C. Flame-cut parts should be annealed
before additional operations in order to reduce the hardness
of the flame-cut edges.
These steels are suitable for welding only when they are in
the annealed or normalized condition. After welding, they
have to be heat treated to obtain the desired strength. The
gas tungsten arc, the gas metal arc, the shielded metal arc,
and the gas welding process are all used for welding these
steels. The composition of the filler metal is designed to
produce a weld deposit that responds to a heat treatment in
approximately the same manner as the base metal.
In order to avoid brittleness and the possibility of cracks
during welding, relatively high preheat and interpass
temperatures are used. Preheating is in the order of
316°C. Complex weldments are heat treated immediately
after welding.
Aircraft engine parts, aircraft tubular frames, and racing
car frames are made from AISI 4130 tubular sections.
These types of structures are normally not heat treated after
welding.
Medium-Alloy Hardenable Steels
These steels are used largely in the aircraft industry for
ultra-high-strength structural applications. They have carbon
in the low to medium range and possess good fracture
toughness at high-strength levels. In addition, they are air
hardened, which reduces the distortion that is encountered
with more drastic quenching methods. Some of the steels in
this group are known as hot work die steels and another
grade has become known as 5Cr-Mo-V aircraft quality steel.
These steels are available as forging billets, bars, sheet,
strip, and plate.
There is another type of steel in this general class which is
a medium-alloy quenched and tempered steel known as
high-yield or HY 130/150. This type of steel is
used for submarines, aerospace applications, and pressure
vessels, and is normally available as plate. This steel has
good notch toughness properties at 0°C and below. These
types of steels have much lower carbon than the grades
mentioned previously.
When flame cutting or welding the aircraft quality steels,
preheating is absolutely necessary since the steels are air
hardening. A preheating on 316°C is used before flame
cutting and then annealed immediately after the flame-cutting
operation. This will avoid a brittle layer at the flame-cut
edge, which is susceptible to cracking.
These types of steel should only be welded in the annealed
condition. The steel should be preheated to 316°C and
this temperature must be maintained throughout the welding
operation. After welding, the work must be cooled slowly.
This can be done by post heating, or by furnace cooling. The
weldment is then stress relieved at 704°C and air cooled
to obtain a fully tempered microstructure suitable for
additional operations. It is usually annealed, after all
welding is done, prior to final heat treatment. The filler
metal should be of the same com-position as the base metal.
The gas tungsten arc and gas metal arc processes are most
widely used. However, shielded metal arc welding, plasma arc,
and electron beam welding processes can be used.
The medium-alloy quenched and tempered high-yield strength
steels are usually welded with the shielded metal arc, gas
metal arc, or the submerged arc welding process. The filler
metal must provide deposited metal of a strength level equal
to the base material. In all cases, a low-hydrogen or
no-hydrogen process is required.
For shielded metal arc welding the low-hydrogen electrodes of
the E-13018 type are recommended. Electrodes must be
properly stored. In the case of the other processes,
precautions should be taken to make sure that the gas is dry
and that the submerged arc flux is dry. By employing the
proper heat input-heat output procedure yield strength and
toughness are maintained. Preheating should be at least at
38°C for thinner materials. For heavier materials
preheating temperature has to be higher.
The heat input should be such that the adjacent base metal
does not become overheated while the heat output is sufficient
to maintain the proper microstructure in the heat-affected
zone. There may be some softening in the intermixing zone.
The properties of welded joints that are properly made will
be in the same order as the base metal. Subsequent
heat-treating is usually not required or desired.
High-Alloy Hardenable Steels
The steels in this group develop high strength by standard
hardening and tempering heat treatments. The steels possess
extremely high strength in the range of 1240 MPa yield and
have a high degree of toughness. This is obtained with a
minimum carbon content usually in the range of 0.20%;
however, these steels contain relatively high amounts of
nickel and cobalt, and they are sometimes called the
9 Ni-4 Co steels. These steels also contain small
amounts of other alloying elements.
They are normally welded in the quenched and tempered
condition by the gas tungsten arc welding process. No
post-heat treatment is required. The filler metal must match
the analysis of the base metal.
High-Nickel Maraging Steels
This type of steel has relatively high nickel, and low carbon
content. It obtains its high strength from a special heat
treatment called maraging. These steels possess an
extraordinary combination of ultra-high-strength and fracture
toughness and at the same time are formable, weldable, and
easy to heat treat. There are three basic types: the steels
with 18% nickel, 20% nickel, and 25% nickel. These steels are
available in sheet, forging billets, bars, strip, and plate.
Some are available as tubing.
The extra special properties of these steels are obtained by
heating the steel to 482°C and allowing it to cool to
room temperature. During this heat treatment all of the
austenite transforms to martensite. The heating time at the
482°C temperature is extremely important and usually is
in the range of three hours. The steels derive their strength
while aging at this temperature in the martensitic condition
and for this reason are known as maraging steels.
These steels are supplied in the soft or annealed condition.
They can be cold worked in this condition and can be flame
cut or plasma arc cut. Plasma arc cutting is preferred.
These steels are usually welded by the gas tungsten arc or
the gas metal arc welding process. The shielded metal arc
and submerged arc process can also be used with special
electrode-flux combinations. The filler metal should have
the same composition as the base metal. In addition, the
filler metal must be of high purity with low carbon. Preheat
or postheat is not required; however, the welding must be
followed by the maraging heat treatment which produces weld
joints of an extremely high strength.
Martensitic Stainless Steels
These steels are of the straight chromium type, such as
AISI 420. They contain 12-14% chromium and up to 0.35%
carbon. This composition combines stainlessness with high
strength. Numerous variations of this basic composition are
available, all of which are in the martensitic classification.
This type of steel has been used for compressor and turbine
blades of jet engines and for other applications in which
moderate corrosion resistance and high strength are required.
The strength level of these steels is obtained by a quenching
and tempering heat treatment. They can be obtained as sheet,
strip, tubing, and plate. The compositions are also used for
castings. These steels can be heat treated to strengths as
high as 1750 MPa yield strength.
These stainless steels can be flame cut by the powder cutting
system normally used for flame cutting stainless steels. They
can also be cut with the oxy-arc process. Flame cutting should
be done with the steel in the annealed condition. Most grades
should be preheated to 316°C because they are air
hardenable. They should be annealed after cutting to restore
softness and ductility. These materials can also be cold
worked in the annealed condition.
The martensitic stainless steels can be welded in the annealed
or fully hardened condition, usually without preheat or
postheat. The gas tungsten arc welding process is normally
used. The filler metal must be of the same analysis as the
base metal. Following welding the weldment should be annealed
and then heat treated to the desired strength level.
Semiaustenitic Precipitation-Hardenable Stainless Steels
The steels in this group are chrome-nickel steels that are
ductile in the annealed condition but can be hardened to high
strength by proper heat treatment. In the annealed condition
the steels are austenitic and can be readily cold worked. By
special heat treatment the austenite is transformed to
martensite and later a precipitant is formed in the
martensite. The outstanding extra high strength is obtained by
a combination of these two hardening processes.
The term semi austenitic type was given these steels to
distinguish them from normal stainless steels. They are also
called precipitation hardening steels or PH steels.
The heat treatment for these steels is based on heating the
annealed material to a temperature between 927°C and
954°C, followed by a tempering or aging treatment in the
range of 454-593°C. These steels are available as billets,
sheets, tubing, and plates.
These steels are normally not flame cut. Welding is performed
using the gas tungsten arc or the gas metal arc welding
process. The shielded metal arc welding process is rarely
used. The filler metal should have the same composition as
the base metal. No preheat or postheat is required if the
parts are welded in the annealed condition. After welding,
the steel has to be heat-treated to develop optimum strength
levels.
However, there is a loss of joint strength due to heating of
the heat-affected zone above the aging temperature. In view
of this, it is not possible to produce a 100% efficient
joint. Extra reinforcing must be utilized to develop
full-strength joints. These steels are also brazed using
nickel alloy filler metal.
When welding on any of these high-strength steels, weld
quality must be of the highest degree. Root fusion must be
complete, and there should be no undercut or any type of
stress risers. The weld metal should be free of porosity
and any weld cracking is absolutely unacceptable. All
precautions must be taken in order to produce the highest
weld quality.
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