Tool Steels
•
A TOOL STEEL is any steel used to make tools for cutting, forming, or
otherwise shaping a material into a part or component adapted to a definite
use.
Composition
•
The earliest tool steels were simple, plain carbon steels, but by the
time, many complex, highly alloyed tool steels were developed.
•
Among other elements, relatively large amounts of tungsten, molybdenum,
vanadium, manganese, and chromium are added.
Properties
•
The alloying additions make it possible to meet increasingly severe service
demands and to provide greater dimensional control and freedom from cracking
during heat treatment.
•
all tool steels must be heat treated to develop specific combinations of
wear resistance, resistance to deformation or breaking under high loads, and
resistance to softening at elevated temperatures.
Manufacturing
•
Most tool steels are wrought products, but precision castings can be used
to advantage in some applications.
•
The powder metallurgy (P/M) process is also used in making tool steels.
(It provides a more uniform carbide size and distribution in large sections and
special compositions that are difficult or impossible to produce by melting and
casting and then mechanically working the cast product)
Performance
•
The performance of a tool in service depends on the proper design of the
tool, accuracy with which the tool is made, selection of the proper tool steel,
and application of the proper heat treatment.
•
A tool can perform successfully in service only when all four of these
requirements have been fulfilled.
Applications
•
Many alloy tool steels are also widely used for machinery components and
structural applications in which particularly stringent requirements must be
met, for example, high-temperature springs, ultrahigh-strength fasteners,
special-purpose valves, and bearings of various types for elevated-temperature
service.
Ultrahigh-Strength
Steels
•
STRUCTURAL STEELS with very high strength levels are often referred to as
ultrahigh-strength steels.
•
The designation ultrahigh-strength is arbitrary because no universally
accepted strength level for the term has been established.
•
Also, as structural steels with greater and greater strength have been
developed, the strength range for which the term is applied has gradually
increased.
•
Here we talk about those commercial structural steels capable of a
minimum yield strength of 1380 MPa (200 ksi).
classification
1.
Medium-carbon low-alloy steels,
2.
Medium-alloy air-hardening steels, and
3.
High fracture toughness steels
4.
Maraging steels
5.
Ultrahigh-strength steels of the stainless type (martensitic, martensitic
precipitation hardenable, semiaustenitic precipitation hardenable, and
cold-rolled austenitic steels)
1.
Medium-carbon
low-alloy steels
Composition
•
C= 0.25- 0.50% Si= up to
0.35%, in some grades up to 1.80%
•
Cr, Ni, Mo and V are added in varying amounts in different grades
Grades
•
The medium-carbon low-alloy family of ultrahigh-strength steels includes
AISI/SAE 4130, the higher-strength 4140, and the deeper hardening,
higher-strength 4340. Several grades with modifications in 4340 have also been
developed.
Recent Developments•
No new or distinctly different commercial steels have been added to this
class of steels in recent years.
•
Rather, developmental efforts have been primarily aimed at increasing
ductility and toughness by improving melting and processing techniques and by
using stricter process control and inspection.
•
Steels with fewer nonmetallic inclusions and fewer internal and surface
imperfections are produced by the use of selected raw materials and the
employment of advanced melting techniques such as vacuum carbon deoxidation,
vacuum degassing, electroslag remelting (ESR), vacuum arc remelting (VAR), and
double vacuum melting (vacuum induction melting followed by vacuum arc remelting).
Shaping
•
Medium-carbon low-alloy ultrahigh-strength steels are readily hot forged,
usually at temperatures ranging from 1065 to 1230°C (1950 to 2250 °F).
•
Medium-carbon low-alloy steels are cut, sheared, punched, and cold formed
in the annealed condition.
•
Medium-carbon low-alloy steels are welded in the annealed or normalized
condition and then heat treated to the desired strength.
2.
Medium-Alloy
Air-Hardening Steels
Grades
•
The ultrahigh-strength steels H11 modified (H11 mod) and H13, which are popularly
known as 5% Cr hot-work die steels are common grades of this type.
Applications
•
Besides being extensively used in dies, these steels are widely used for
structural applications, but not as widely as they once were, primarily because
of the development of several other steels at essentially the same cost but
with substantially greater fracture toughness at equivalent strength.
•
Nonetheless, H11 mod and H13 possess some attractive features, e.g. both
can be hardened through in large sections by air cooling.
H11 Modified
•
The H11 mod steel can be heat treated to strengths exceeding 2070 MPa
(300 ksi). It is air hardened, which results in minimal residual stress after
hardening.
Properties
• At high strength levels
(those exceeding 1800 MPa, or 260 ksi), H11 mod has good ductility, impact
strength, notch toughness, and fatigue life, as well as high creep and rupture
strength, at temperatures up to about 650 °C (1200 °F).
Applications
•
It is used for parts requiring maximum levels of strength, ductility,
toughness, fatigue resistance, and thermal stability at temperatures between
−75 and 540 °C (−100 and 1000 °F).
Oxidation Problem
•
At elevated temperatures, parts should be protected from corrosion
(oxidation) by an appropriate surface treatment.
•
Parts to be used at elevated temperatures are commonly protected by
nickel-cadmium plating
•
Alternatively, part surfaces may be protected from oxidation by hot
dipping in aluminum or by applying a heat-resistant paint.
3.
High Fracture
Toughness Steels
•
These are high-strength, high fracture toughness commercial structural
steels capable of a yield strength of 1380 MPa (200 ksi) and a KIc
of 100 MPa. (These steels also exhibit stress corrosion cracking resistance.)
•
The HP-9-4-30 and AF1410 steels are being discussed below.
•
Both these alloys are of the Ni-Co-Fe type and have a number of similar
characteristics.
•
Both are weldable, and the melt practice requires a minimum of VAR.
Control of residual elements to low levels is required for optimum toughness.
•
Ni-Co-Fe alloys are considered more difficult to machine than are alloy
steels.
AF1410 Steel
•
The steel AF1410 was an outgrowth of the U.S. Air Force sponsorship of
the advanced submarine hull steels, the result of which was the development of
the low-carbon Fe-Ni-Co type alloys.
Properties
•
These alloys had significant stress corrosion cracking resistance.
•
By raising the cobalt and carbon content, the ultimate tensile strength
was increased to a typical 1615 MPa (235 ksi).
•
This increase in strength was obtained while maintaining a KIc
value of 154 MPa
•
The AF1410 material is air hardenable in sections up to 75 mm (3 in.)
thick.
•
The preferred melting practice is presently vacuum induction melting
followed by vacuum arc remelting (VIM/VAR).
Forming
•
Obtainable as bar, billet, rod, plate, sheet, and strip
•
Although forgeable to 1120 °C (2050 °F), at least a 40% reduction must be
obtained below 900 °C (1650 °F) to attain maximum properties.
•
Weldability is good using a gas tungsten arc welding (GTAW) process.
Applications
•
The combination of strength and toughness exceeds that of other
commercially available steels, and the alloy has been considered as a
replacement for titanium in certain aircraft parts.
•
AF1410 has been used for aircraft structural components.
4.
Maraging Steels
•
MARAGING STEELS
comprise a special class of high-strength steels that differ from conventional
steels in that they are hardened by a metallurgical reaction that does not
involve carbon.
•
Instead, these
steels are strengthened by the precipitation of intermetallic compounds at
temperatures of about 480 °C (900 °F).
•
The term
maraging is derived from martensite age hardening and denotes the
age hardening of a low-carbon, iron-nickel lath martensite matrix.
Composition
•
Maraging steels
can be considered highly alloyed low-carbon, iron-nickel lath martensites.
These alloys also contain small but significant amounts of titanium.
•
These steels
typically have very high nickel, cobalt, and molybdenum contents and very low
carbon contents(<0.03%).
•
Carbon, in fact,
is an impurity in these steels and is kept as low as commercially feasible in
order to minimize the formation of titanium carbide (TiC), which can adversely
affect strength, ductility, and toughness.
•
Other varieties
of maraging steel have been developed for special applications.
Hardening Mechanism
•
The phase
transformations in these steels can be explained with the help of the two phase
diagrams shown in Fig. 1 , which depict the iron-rich end of the Fe-Ni binary
system.
•
Figure 1 (a) is
the metastable diagram plotting the austenite-to-martensite transformation upon
cooling and the martensite-to-austenite reversion upon heating.
• Figure 1 (b) is the equilibrium diagram showing that
at higher nickel contents the equilibrium phases at low temperatures are
austenite and ferrite.
•
No phase
transformations occur until the Ms temperature, the temperature at which
martensite starts to transform from austenite, is reached.
•
Even very low
cooling of heavy sections produces a fully martensitic structure.
•
Most grades of
maraging steel have Ms temperatures of the order of 200 to 300 °C (390 to 570
°F) and are fully martensitic at room temperature.
•
The martensite
is normally a low-carbon, body-centered cubic (bcc) lath martensite containing
a high dislocation density but no twinning.
•
This martensite
is relatively soft (~ 30 HRC), ductile, and machinable.
Age Hardening
•
The age
hardening of maraging steels is produced by heat treating for 3 to 9 h at
temperatures of the order of 455 to 510 °C
•
With prolonged
aging, the structure tends to revert to the equilibrium phases--primarily
ferrite and austenite.
•
Fortunately, the
kinetics of the precipitation reactions that cause hardening are such that
considerable age hardening occurs before the onset of the reversion reactions
that produce austenite and ferrite.
•
With long aging
times or high temperatures, however, hardness will reach a maximum and then
will start to drop, as shown by the data in Fig. 2
•
Overaging causes
austenite reversion and hence softening.
•
Maraging steels
normally contain little or no austenite after standard maraging heat
treatments.
•
Age hardening in
maraging steels results primarily from the precipitation of intermetallic
compounds.
•
Precipitation
takes place preferentially on dislocations and within the lath martensite to
produce a fine uniform distribution of coherent particles.
•
The major
hardener is molybdenum, which upon aging initially forms Ni3Mo.
•
Further aging
results in the transformation of Ni3Mo to the equilibrium Fe2Mo
phase.
•
Titanium, which
is generally present in maraging steels, promotes additional age hardening
through the precipitation of Ni3Ti.
Properties
•
Commercial
maraging steels are designed to provide specific levels of yield strength from
1030 to 2420 MPa (150 to 350 ksi).
•
Some
experimental maraging steels have yield strengths as high as 3450 MPa (500
ksi).
•
Weldability is
excellent.
•
Fracture
toughness is considerably better than that of conventional high-strength
steels.
Applications
•
Maraging steels
have been used in a wide variety of applications, including missile cases,
aircraft forgings, structural parts, cannon recoil springs, Belleville springs,
bearings, transmission shafts, fan shafts in commercial jet engines, couplings,
hydraulic hoses, bolts, punches, and dies.
•
Maraging steels
have been extensively used in two general types of applications:
Ø Aircraft and aerospace applications, in which the superior mechanical properties and
weldability of maraging steels are the most important characteristics
Ø Tooling applications, in which the excellent mechanical properties and
superior fabricability (in particular, the lack of distortion during age
hardening) are important
•
In many
applications, even though maraging steels are more expensive than conventional
steels in terms of alloy cost, finished parts made of maraging steel are less
expensive because of significantly lower fabrication costs. Therefore, it is
often economics rather than properties alone that determine the use of maraging
steels.
