Ultra high strength steels

1

• Ultra high strength steels

2
ULTRA HIGH STRENGTH STEELS
Introduction in

• Conventional direct hardening steels are
  usually designed ranges of tensile strength,
  which in commercial practice ranges from 75 or
  100 MPa. For example, 850-1100 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
practice.
3
(No Transcript)
4
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.

5

• 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, 0.005S max.
• The process are slow (approximately 1 tonne/h)
  hence relatively high costs are incurred
  (typically 3 times more expensive than the
  conventional condition).

6
Hardening mechanisms of maraging steel

• 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.

Solution hardening in alloy steels
7
(No Transcript)
8
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
9
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.
10
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.

11

• 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.

12

• 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.

13
(No Transcript)
14

• 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.
15
(No Transcript)
16
Thank you!