Maraging Steel Metallurgy

Maraging Steels: A Comprehensive Review

Introduction to Maraging Steels

• Maraging steels are high-strength steels with very low carbon content.
• They utilize substitutional elements to achieve age-hardening in iron-nickel martensites.
• The term 'maraging' combines "martensite" and "age-hardening".
• Initial development occurred in the late 1950s by C. G. Bieber at The International Nickel Company.
• The first two grades were 25% and 20% nickel steels.

Composition and Properties of Maraging Steels

• Alloy A (25% Ni steel):

  • Semi-austenitic composition.
  • After solution-annealing, remains austenitic at room temperature because its Ms temperature is sufficiently low.
  • Transformation to martensite requires a two-step heat treatment:
    • Ausaging: Heat at approximately $700°C$ ($1300°F$) to precipitate nickel-rich intermetallic compounds in austenite, raising the Ms temperature.
    • Refrigeration: Cool to $-77°C$ ($-106°F$) for complete transformation to martensite.
  • Alternatively, transform by cold working at least 25% followed by refrigeration.
  • Final hardening achieved by maraging at $425-485°C$ ($800-900°F$).

• Alloy B (20% Ni steel):

  • Ms temperature around $200°C$ ($400°F$).
  • Completely transforms to martensite after solution-annealing.
  • Hardening requires only a maraging heat-treatment in the $425-485°C$ temperature range.

• Alloys C, D, and E (18Ni (200), 18Ni (250), and 18 Ni (300) grades):

  • Based on the discovery by Decker, Eash, and Goldman (1960) that cobalt and molybdenum effectively harden Fe-Ni martensites.
  • Contain an 18% nickel base with varying cobalt, molybdenum, and titanium levels.
  • Achieve nominal strength levels of 200, 250, and 300 ksi after maraging, respectively.
  • Titanium acts as a supplemental hardener; the primary strengthening effect results from the combination of cobalt and molybdenum.

• Specialized Alloys:

  • Alloy F: Composition with greater resistance to austenite reversion, suitable for elevated-temperature magnetic applications.
  • Alloy G: Recent composition achieving a 350 ksi strength level.
  • Alloy H: A cast alloy.
  • Alloy I (12-5-3 maraging steel): Does not use cobalt and molybdenum for hardening; designed for high fracture toughness at strengths around 180 ksi.

• General Characteristics:

  • Carbon content specified at 0.03 wt.-% maximum in all alloys.
  • Titanium serves as a hardener and refining agent, tying up residual carbon.

Heat Treatment and Processing

• Solution Annealing: Typically performed for 1 hour at $815°C$ ($1500°F$), though other temperatures or multiple annealing steps are used in some cases.
• Martensite Transformation: Alloys completely transform to martensite upon cooling to room temperature after annealing.
• Hardenability: High nickel content and virtual absence of carbon ensure hardenability, making the cooling rate after annealing unimportant.
• Machinability: Alloys in the as-annealed condition have a hardness around Rockwell C 30 and can be readily machined or fabricated.
• Maraging: Achieved generally using a treatment of 3 hours at $485°C$ ($900°F$).
• Dimensional Stability: Minimal dimensional changes during age-hardening allow for machining finished pieces before hardening.
• Weldability: Alloys are readily weldable.
• Resistance to Embrittlement: Possess superior resistance to hydrogen embrittlement and stress-corrosion cracking compared to high-strength low-alloy steels.
• Toughness: Exhibit distinctly better toughness at high strength levels.

Phase Transformations in Maraging Steels

• Iron-Nickel Phase Diagram:

  • Essential for understanding maraging steel behavior; two diagrams are considered: equilibrium and metastable.
  • Equilibrium diagram (Owen and Liu) indicates that for alloys containing 3-30% Ni, the equilibrium phases at lower temperatures are ferrite and austenite.
  • In practice, cooling an 18% nickel alloy from the austenite field results in incomplete decomposition into equilibrium austenite and ferrite.
  • Instead, austenite transforms to martensite with a body-centered cubic (b.c.c.) crystal structure.

• Martensite Transformation Temperatures:

  • Shown as a function of nickel content in the metastable equilibrium diagram (Jones and Pumphrey).
  • Heating martensite below the As temperature (start of austenite transformation) causes decomposition into equilibrium austenite and ferrite.
  • Reversion reaction rate is slow enough at around $485°C$ ($900°F$) to allow significant precipitation-hardening before it predominates.
  • Heating above the As temperature transforms martensite back to austenite via a shear reaction.
  • Reversion often occurs even with fast heating rates, influencing the shear reaction.

• Limitations of the Iron-Nickel Diagram:

  • The diagram is oversimplified; recent work reveals varied transformations.
  • Ferrite forms at slower cooling rates and martensite at higher rates in the 0-10% Ni range.
  • Increasing nickel content lowers the cooling rate needed for martensite formation; alloys with ~10% nickel form completely martensitic structures even with slow cooling.
  • This martensite is termed "massive martensite".

Martensite Morphology

• Massive Martensite:

  • Found in the ~10-25% Ni range, varying with cooling rate, annealing temperature, and interstitial content.
  • Beyond ~25% Ni, twinned and surface martensite morphologies appear.
  • Massive martensite is most relevant to maraging steels.
  • Structure consists of elongated laths or platelets with high dislocation density.
  • Dislocations lie predominantly in [111]b.c.c. directions and are screw in nature (Patterson and Wayman).
  • Platelet interfaces are typically wavy.
  • Adjoining platelets are not generally twin-related.
  • The habit plane is [0.463, 0.532, 0.708]γ (100 from (111)γ).
  • The Orientation relationship is believed to be Kurdjumov-Sachs

• Factors Influencing Martensite Morphology:

  • Miller and Mitchell's work on Fe-7% Co-5% Mo-0.4% Ti alloys shows that massive martensite forms with nickel contents up to 23%, and twinned martensite at higher nickel contents.
  • Lack of visible surface shears in 18 Ni (250) steel sometimes raises doubts about massive martensite formation.
  • Transmission microscopy suggests that all maraging steels, except 25% nickel steel, typically have massive martensite matrix structures.
  • Ms temperature influences martensite type; lower Ms temperatures favor twinned structures.
  • Massive martensite matrices may offer better toughness after aging than twinned martensite.
  • The need to avoid twinned martensite and untransformed austenite restricts alloying additions to Fe-Ni bases.

Properties of Massive Martensites

• Deformation Behavior:

  • Studies on 18% Ni binary and Fe-Ni-Cr ternary massive martensites show similar behavior.
  • Yield strengths are around 100 ksi.
  • Work-hardening, temperature and strain-rate dependence of flow stress, and activation volumes/energies are consistent with iron or low-alloy steels.
  • Petch slope values are on the low side compared to low-alloy steels.
  • Deformation characteristics resemble those of highly alloyed b.c.c. iron.
  • 18% nickel alloy shows good tensile ductility even in liquid hydrogen.
  • Higher nickel levels in Fe-Ni-Cr ternary alloys improve impact properties at sub-zero temperatures; chromium is also slightly beneficial.
  • Localized slip bands are noticeable in the 18% Ni alloy, possibly due to the dislocation structure introduced by the martensite transformation.

• Heat Treatment Effects:

  • Heating to $400°C$ ($750°F$) increases elastic limits and slightly increases yield and ultimate tensile strengths.
  • Small decreases in residual microstress and electrical resistivity are observed after heating 25% nickel alloys.
  • Increases in elastic limits in Fe-Ni-Cr alloys are inversely related to Ms temperatures.
  • Changes in properties may be due to precipitation of residual carbon or nitrogen or recovery reactions relieving residual stresses from the martensite transformation.

Ausaging Reactions

• Precipitation:

  • Ausaging 25% Ni steel for 4 hours at $705°C$ ($1300°F$) produces small, spherical precipitates containing 70% Ni, 20% Ti, and small amounts of Fe, Al, and Si (Reisdorf).
  • Pitler and Ansell's study (28% Ni, higher Ti, Al, and Nb) observed 50-Å-diameter precipitates after ausaging for 0.1 hours at $705°C$ ($1300°F$). These grew proportionally to (time)^1/3 up to ~150 Å and then grew at a slower rate. After 64 hours, uniformly distributed 250-Å-diameter spheres and cellular precipitates at grain boundaries were observed. Both precipitates were identified as γ'-Ni3(Ti, Al).
  • The spherical particles were randomly oriented within the austenite grains.
  • Garwood and Jones also observed spherical γ' precipitates within the matrix and cellular γ' as well as η, the hexagonal form of Ni3Ti, in grain boundaries after overaging.
  • The spherical γ' particles distinctly showed a preferred orientation.
  • Hornbogen and Mayer found oriented γ' precipitates after ausaging Fe-Ni-Al and Fe-Ni-Al-Ti alloys.
  • Variations in Ms temperature depend on ausaging conditions.

• Ms Temperature Variations:

  • Alloying elements removed from solution lead to an expected rise in Ms temperature.
  • Ausaging at approximately $600°C$ ($1115°F$) initially decreases Ms, rising only after prolonged ageing times.
  • Ageing at $700°C$ ($1295°F$) results in a continuous increase in Ms (Hornbogen and Mayer).
  • Hornbogen and Mayer found that small particles of face-centered cubic (f.c.c.) copper or γ' precipitated in austenite were sheared to a b.c.c. crystal structure during martensite transformation.
  • The decrease in Ms after short ageing times at $600°C$ is attributed to the energy required to shear these particles.
  • After longer ageing, coarser precipitates in austenite are no longer transformed, causing the removal of alloying elements from solid solution to predominate, and the Ms temperature rises.
  • Mechanical stabilization or destruction of martensite embryos by precipitation may also influence Ms temperature.

• Martensite Structure and Mechanical Properties:

  • Variations in Ms temperature lead to corresponding changes in martensite structure.
  • Low Ms temperatures produce twinned martensite, while higher Ms temperatures after further ausaging result in massive martensite.
  • Cold work + refrigeration yields superior mechanical properties compared to ausaging + refrigeration (Decker et al.).
  • The former produces massive martensite, while the latter gives twinned martensite.
  • Twinned martensite has poorer fracture resistance than massive martensite in hydrogen-charged samples (Bonizewski and Baker).

Maraging after Ausaging

• Precipitate Evolution:

  • During maraging after ausaging and refrigeration, γ' precipitates begin to dissolve and are replaced by η phase (Pitler and Ansell).
  • η phase forms as thin plates in a Widmanstatten pattern.
  • Garwood and Jones observed that γ' particles elongated during maraging, proposing that a Ni3Ti precipitate with the f.c.c. structure of γ' was nucleated at the original γ' particles.
  • Reversion of martensite to austenite also appeared to nucleate at γ' particle surfaces.
  • η precipitation was observed only in segregated regions with fewer γ' particles.
  • Differences between results are attributed to variations in alloy chemical compositions.

• Hardness Contribution:

  • Prior ausaging can increase the Vickers hardness of as-formed martensite by up to ~100 points.
  • During subsequent maraging, hardness differences between ausaged and non-ausaged specimens generally decrease.
  • Prior ausaging makes a significant contribution to final strength after maraging.

Maraging Reactions

• Alloying Elements:

  • Many substitutional alloying elements can cause age-hardening in Fe-Ni martensites, with varying degrees of hardening based on nickel content, ageing kinetics, and reversion reactions.
  • Strong interactions exist between specific element combinations, such as cobalt and molybdenum.

• Classification:

  • Strong hardeners: Beryllium (Be) and Titanium (Ti).
  • Moderate hardeners: Aluminum (Al), Niobium (Nb), Manganese (Mn), Molybdenum (Mo), Silicon (Si), Tantalum (Ta), Vanadium (V), and Tungsten (W).
  • Weak hardeners: Cobalt (Co), Copper (Cu), and Zirconium (Zr).

• Precipitate Identification:

  • Extensive research on precipitate formation during maraging reactions.
  • Phase-identification studies are summarized in Table II.
  • Common technique involves studying diffraction patterns from extracted precipitate particles.
  • Challenges include difficulty extracting small particles, discrepancies in diffraction patterns, and potential confusion from extraneous sources.
  • Supplemental techniques include selected-area electron-diffraction patterns from thin foils, microprobe analysis, Mössbauer spectroscopy, and analysis of angles between diffraction spots.

• Discrepancies and Trends:

  • Variations in identified precipitates in alloys with similar compositions due to inherent uncertainties in phase identifications.
  • Ageing for several hours at approximately $485°C$ ($900°F$) typically produces Ni3Mo in alloys containing ~18% Ni.
  • Longer times or higher temperatures result in Fe2Mo Laves or μ phases.
  • Alloys with higher molybdenum and/or lower nickel levels tend to form Fe2Mo or Fe7Mo6.
  • Conventional maraging heat-treatment produces a metastable Ni3Mo precipitate, replaced by Fe2Mo or μ at longer times and/or higher temperatures.
  • Mössbauer study supports the sequence of metastable and more stable molybdenum precipitates (Marcus et al.).
  • Depletion of alloying elements from the martensite matrix is complete after 3 hours at $480°C$.(895°F)
  • Good lattice fit between A3B precipitates with a hexagonal close-packed (h.c.p.) crystal structure and the b.c.c. martensite matrix favors A3B compound precipitation (Detert).
  • While NisMo may initially precipitate due to matrix fit, increased coherency stresses may limit its growth and lead to equilibrium precipitate nucleation.

• Titanium Precipitation:

  • In 20 and 25% nickel steels, titanium is the primary hardener, forming η-Ni3Ti.
  • In 18Ni (250) type alloys, titanium is a supplemental hardener.
  • The role of titanium in these alloys is less certain.
  • Various investigators have suggested that titanium precipitates as η-Ni3Ti. Distinguishing it from Ni3Mo is challenging due to their similar structures.
  • Chilton and Barton found that titanium precipitated as a σ phase.
  • Some titanium may be present in the molybdenum precipitate, i.e., as Ni3(Mo, Ti).

• General Observations:

  • A3B-type compounds are favored by higher nickel or nickel + cobalt contents.
  • At lower levels, precipitates are typically A2B (Laves) or A7B6 (μ) phases.
  • Higher electron: atom ratios with higher nickel and/or cobalt may influence this trend.
  • Chemical analyses reveal that precipitate compositions are not simply those described by structural formulas, containing considerable iron and occasional small amounts of other alloying elements.
  • Cobalt is usually present only in small quantities or not at all.
  • Mössbauer studies show no significant cobalt precipitation during maraging.

• Microscopic Studies:

  • Transmission electron microscopy studies reveal structures consisting of rod or ribbon-like Ni3Mo precipitates and spherical titanium-containing σ phase particles.
  • Dislocation rearrangement and decreased dislocation density occur during early maraging (recovery reaction).
  • Pre-precipitate zones may form during early age-hardening.
  • Precipitates often nucleate at dislocations or martensite platelet boundaries, distributing uniformly, and coarse precipitates at grain boundaries are not typically seen.

• Precipitate Morphology:

  • After ageing for several hours at ~$485°C$ ($900°F$), precipitate particles typically reach sizes of several hundred Angstroms.
  • Different morphologies include needles, plates, and spheres, which can vary significantly with slight changes in composition or heat-treatment.
  • Precipitates often have a preferred orientation and may be coherent with the matrix, with detectable strain fields around them.
  • Chilton and Barton noted that Ni3Mo precipitate orientation changes with the secondary precipitate; Ti- or V-bearing σ phase leads to one orientation, while Fe3Al precipitates cause a different orientation.
  • Garwood and Jones suggest that preferred precipitate orientation arises from dislocation orientation within the martensite matrix.

The Cobalt/Molybdenum Interaction

• Synergistic Hardening:

  • The combination of cobalt and molybdenum results in a much greater hardening effect than the sum of individual additions (Decker, Eash, and Goldman).
  • Although Banerjee and Hauser did not observe this effect, the data of other investigators show this general result.
  • Age-hardening can be produced in ternary Fe-Ni-Co alloys, achieving high strengths at higher nickel and cobalt levels.
  • Strengths increase approximately linearly with increasing cobalt concentration.

• Mechanisms:

  • Mihalisin stated that neutron diffraction indicates that B2-type long-range ordering develops during maraging a 22.7% Ni-19.3% Co ternary alloy.
  • Two types of precipitate structurally identical to austenite were observed after maraging a 19.0% Ni-28.5% Co composition.
  • Strengthening in higher cobalt alloys is due to a combination of ordering and precipitation-hardening.
  • In alloys of 18% Ni-8% Co composition, no evidence of long-range ordering or precipitation-hardening has been found; however, maraging produces a strength increment of ~25 ksi.
  • The strengthening in 18% Ni-8% Co alloys may be due to low degree long-range ordering that may escape detection in neutron-diffraction analysis.

• Cobalt's Role:

  • Floreen and Speich found that adding 8% Co to 18% Ni-X ternary and 18% Ni-8% Co-X quaternary alloys (where X is Al, Be, Mn, Mo, Nb, Si, or Ti), increased the yield strength by ~20-45 ksi, except for molybdenum-containing alloys.
  • In molybdenum-containing alloys, the strength was increased up to 75 ksi by the cobalt addition.
  • Transmission electron microscopy suggests cobalt addition results in a finer dispersion of precipitates in molybdenum-bearing alloys.
  • Cobalt may lower the solubility of molybdenum in the matrix, increasing the amount of molybdenum precipitate.

• Peters and Cupp's Study:

  • Electrical-resistivity measurements showed that resistivity was directly proportional to molybdenum in solid solution, while changes in cobalt content had a negligible effect on resistivity.

• Alternative Explanation:

  • Cobalt may alter the dislocation structure of the martensite matrix, providing more uniform nucleation sites for subsequent precipitation.

Maraging Kinetics

• Reactions During Maraging:

  • A number of reactions can take place during maraging, and complete unravelling of these individual reactions has not been possible.
  • Up to the time when the reversion of the martensite matrix to austenite becomes a major factor, or roughly up to the time when peak hardness is attained, the kinetics can be studied reasonably well.
  • As noted before, many investigators have seen some dislocation rearrangement and loss during the initial stages of maraging.

• Recovery Reaction:

  • Miner et al studied the kinetics of this recovery reaction by means of internal-friction techniques and found a rapid initial drop in clamping that was temperature-independent and similar in general appearance to the recovery observed in cold-worked metals.
  • They concluded that the recovery was due to athermal dislocation glide, and was ended by precipitation.

• Precipitation-Hardening:

  • Precipitation-hardening takes place very rapidly.
  • In the 18 Ni (250) steel the Vickers hardness was raised by the order of 100 points after several minutes' ageing at $485°C$ ($900°F$).
  • Comparable hardness increases were observed in a number of Fe-18% Ni-base ternary alloys after similar heat-treatments.

• Resistivity Changes:

  • Small but real changes in resistivity were noted after the shortest ageing times.
    *Peters and Cupp concluded that the incubation time for age-hardening was zero and is consistent with Miner et al

• Molybdenum Precipitate Reversion:

  • Peters and Cupp showed that there was a pronounced discontinuity in the ageing behavior of the quaternary Fe-18% Ni-7.8% Co-5.7% Mo alloy at 450° C.
  • They attributed this to possible matrix precipiate.

• Isothermal Ageing Kinetics:

  • Isothermal ageing kinetics could be expressed quite well by a relationship of the type: \x/x0 = K \cdot t^n, where x is either the hardness or electrical resistivity at time t, $x</em>0$ the as-annealed value, and K and n are constants.
  • Values of the time constant n were of the order of 0.2-0.4; i.e., well below the n value of 0.5 for the idealized case of diffusion-controlled growth of platelets.
  • Nominal values of the activation energies were fairly low (30-40 kcal/mole), thus well below those commonly observed for substitutional-element diffusion in ferrite.

Effects of Working Before Maraging

• Warm Working:

  • Ausforming type warm-working experiments, i.e. deformation of the austenite preceding the transformation to martensite,produced only very minor improvements in the final strength.

  • A rather noticeable loss of toughness in a direction transverse to the original working direction is apparent

  • The lack of any significant hardening is interpreted to the very low carbon contents of the maragin steels.

  • Cold Working:

  • Cold working the martensite before maraging shows substantial strength increase.

  • Cold working tends to increase the initial rate of hardening somewhat.

Austenite Formation

• Diffusion-Controlled Decomposition:

  • The metastable iron-nickel martensite matrix decomposes by a diffusion-controlled reaction forming ferrite and austenite during prolonged elevated temperatures.
  • Rate of austenite formation is composition-dependent.

• Nickel Levels Effect:

  • Increasing nickel accelerates austenite formation.

• Additional Alloying Elements Effect:

  • Impact on reversion reaction depends whether the element stabilizes austenite or ferrite.
  • Titanium retards reversion via Ni3Ti formation, lowering nickel content in matrix.
  • Molybdenum promotes reversion via Ni3Mo dissolution and Fe2Mo formation.

Relationship of Structure to strength and toughness

• The primary attribute of maraging steels is their excellent combination of strength and toughness.
• This is true of the 180/0nickel grades, which in general have a toughness superior to that of the other grades at high strength levels.