Maraging Steel1. Introduction and General Characteristics: Maraging steels are characterized with superior strength

combined with excellent toughness properties and weldability. without loss of malleability. OR

Maraging steels are carbon free iron-nickel alloys with additions of cobalt, molybdenum, titanium and aluminium.

The term maraging is derived from the strengthening mechanism, which is transforming the alloy to martensite with subsequent age hardening. (Martensite + Ageing)

• special class of steel contain No-carbon but provides “ultra-high-strength”

• Their strength comes from precipitation of intermetallic compounds like (Mo, Ti & Al)Ni3 “NOT FROM CARBON”.

• High strength is maintained up to 350 oC. …? How?

cont…• Since ductile Fe-Ni martensites are formed upon cooling,

cracks are non-existent or negligible. • These steels can be nitrided to improve case hardness.• C is considered as an impurity element in these steel and it

should be kept below 0.03%.2. Application: • Considered as Military grade … crankshafts, gears, and the

firing pins of automatic weapons that cycle from hot to cool repeatedly under substantial load.

• Missile Nose. aerospace, e.g. undercarriage parts and wing fittings, tooling & machinery , e.g. extrusion press rams and mandrels in tube production, gears

• Ordnance components and fasteners.

3. Types of Maraging Steels Well known grades 18%, 20% & 25%

4. Production of Maraging SteelE.A.F/ V.I.M

L.F

V.D/V.C.D

I.C

V.A.R

Shaping (Rolling/Forging )

Solution Treatment + Ageing

Nitriding (If Required)

Most Important Step

5. Heat Treatment of Maraging Steel:

Solution Treatment

+ Aging

6. Physical Metallurgy of Maraging Steel /• Effects a) Ni = Austenitic Stabilizing Element, Increases hardenability, resistant to fatigue &

corrosion b) Co/Mo = Retain hardness at high temperature, good wear resistant.c) Ti = Stabilize Ferrite, Refine Grain structure, raises creep strength.d) Al = stabilize ferrite, form nitride, refine grain size.

Element 200 250 300 350Substitutional Element Ni 17-19 17-19 18-19 18-19Substitutional Element Co 8-9 7-8.5 8.5-9.5 11.5-12.5Substitutional Element Mo 3-3.5 4.5-5.2 4.6-5.2 4.6-5.2Substitutional Element (Partially Soluble)

Ti 0.15-0.25 0.3-0.5 0.5-0.8 1.3-1.6

Substitutional Element Al 0.05-0.15 0.3-0.5 0.5-0.8 1.3-1.6Substitutional Element Fe Balance Balance Balance Balance

• 18%Ni Maraging Steel Grades

6.1 Fe-Ni Phase Diagram

• steels are solution treated at 820 oC , to absorb all precipitates or alloying elements & produce uniform austenitic structure.

• upon cooling in air/quenching, an Fe-Ni BCC Martensite is formed instead of ordinary Tetragonal Martensite (Fe-C).

• This is of lath-like BCC form, softer & tougher than ordinary martensite but heavily dislocated martensite.

(means high dislocation density = high energy = means favorable site for precipitation)• Upon ageing at 480oC for 3 or more hours, coherent

precipitates of intermetallic compound (Mo, Ti & Al)Ni3 are formed.

• The main function of “Co” seems to produce more sites for the nucleation of (Mo, Ti & Al)Ni3 precipitates. Or Co reduces the solubility of Ti, Mo & Al in the matrix as a result theses increases the volume fraction of rich precipitate.

• If the precipitation hardening temperaure exceeds 500oC, over-aging results the strength begin to fall due to reversion of the austenite.

• similarly increases the time for aging the same effect will be produce.

Why Ni is selected for Maraging steels ? How it can with stand high strength at high temperature?

• High-nickel steels introduces considerable thermal hysteresis in the polymorphic transformation.

• steel containing 15% nickel will not begin to transform on cooling until a temperature of about 250oC has been reached, when martensite begins to form.

• On re-heating the structure, however, martensite does not begin to change to austenite until a temperature approaching 600oC.

• This hysteresis, or lag, in transformation is fundamental to the use of nickel in maraging steels.