Transcript Slide 1
Ultra high strength
steels
RG
1
Introduction in
ULTRA HIGH STRENGTH
STEELS
Conventional direct
hardening steels are
usually designed ranges
of tensile strength, which
in commercial practice
ranges from 75 or 100
MPa. For example, 8501100 MPa.
General effect of Tempering Temperature on
the Room Temperature Mechanical
Properties of Steels Initially Quenched to be
Almost Completely Martensitic.
In commercial engineering applications, the tensile strength is usually
limited to a maximum of 1250 MPa, with a few exceptions to 1500MPa.
Above 1500 MPa, steels are considered to be ultra-high tensile with
2200MPa being a limit for conventional quench/temper heat treatment
RG
2
practice.
RG
3
Remelted Ultra-High Strength Steels
Electro-Slag Remelting (ESR) and Vacuum Arc Remelting (VAR) are
processes often used to improve the purity and reduce the inclusion
content of ultra-high strength alloy steels. They involve total remelting of
a consumable “Electrode” in the form of a specially prepared ingot of the
steel quality to be remelted.
ESR is a reactive process whereby sulfur is reduced and was used
for that purpose before ladle technology was developed to its current
level. Inclusions are removed by chemical and physical means. Even
though the inclusion content is less, the oxygen content is higher than
air melted steels. Nitrogen levels do not change significantly.
Hydrogen can increase if an inert atmosphere is not used but
steelmakers know how to remove it later by heat treating the product.
RG
4
VAR removes oxygen, while hydrogen and nitrogen can remove
sulfur, although the feedstock ingots for the process are often made
to very low sulfur levels, 0.005%S max.
The process are slow (approximately – 1 tonne/h) hence relatively
high costs are incurred (typically 3 times more expensive than the
conventional condition).
RG
5
•
•
•
Solution hardening. Except for nitrogen,
which dissolves as an interstitial like
carbon, all other suitable elements will
always be of the substitutional solid
solution type.
Precipitation hardening. Either by
forming finely dispersed hard and small
carbides of the alloying elements, or by
influencing the cementite formation to
occur in fine particles, or by producing
precipitates of compounds of the
alloying elements (e.g. borides, or
intermetallic phases), or by all of the
above.
Grain size reduction. You may produce
small grains (i.e. from a martensitic
transformation which rips on i "apart" in
many grains), and/or keep small grains
small by keepig grain boundaries from
moving (i.e. grains from growing) by
precipitating suitable elements there
(without making the grain boundary
brittle, of course). This will always lead
to hardening, too.
RG
Hardening mechanisms of maraging
steel
Solution hardening in alloy steels
6
RG
7
Processing of Ultra-High Strength Steels
Processing of ultra-high strength involves tighter disciplines than are normally
adopted for lower strength direct hardening alloy steels.
Processing stages for ultra-high strength steels
RG
8
Maraging steels
Maraging steels are special ultra-high strength steels that achieve
high levels of strength and toughness quite differently from
conventional alloy steels. Whilst the structure is low carbon (lath)
martensite, their high hardness and strength are produced by the
precipitation of intermetallic compounds throughout the matrix.
Tensile strength
Fracture Toughness of Maraging Steels and
Other High-Strength Martensitic Grades.
RG
9
Tipical chemical composition and mechanical
properties of maraging steels
Maraging steels have very low carbon contents
(0.01-0.03 max.) and restricted silicon and manganese
levels. The major alloying elements are nickel, cobalt,
molybdenum and titanium. Like other ultra-high strength
steels, they are invariably vacuum arc remelted.
RG
10
This iron-based family of steels are based on a
composition of iron and 18 % nickel with
additions of cobalt, molybdenum and titanium
plus other elements. The most frequently
specified 'C' grades contain a significant amount
of cobalt. There are also 'T' (titanium) grades
which contain no cobalt and have a lower
molybdenum content and a greater addition of
titanium compared with the corresponding
strength 'C' grades.
RG
11
The structure of the alloys are formed of fine martensite
which then undergoes an aging process (precipitation
hardening) giving them the name 'mar-aging' steels.
Maraging steels are characterised by ultra-high strength,
simple aging treatment which minimises distortion, high
levels of toughness, moderate corrosion resistance
(similar to that of standard martensitic steels), good
machinability (usually in the annealed condition) and
good weldability.
RG
12
RG
13
APPLICATIONS OF MARAGING STEELS
Applications of are always where the unique strength/weight ratio is vital; this is
done in order to justify the very high cost of these steels.
They are used in ultra-high strength aircraft components and find a strategic
military role in lightweight bridges, etc.
Formula One Racing structural components are often made from these steels.
In addition to superb mechanical proporties there is little distortion experienced
during the aging process.
Maraging steels are
diverse and include missile
casings, tooling, ordnance
mounting blocks, high
performance autosport
components, couplings,
bearings, load cells,
landing gear components,
transmission shafts, jet
engine components and
helicopter drive shafts.
RG
14
RG
15
RG
16