Maraging Steels (Properties, Microstructure & Applications)

Maraging steel is used in aircraft, with applications including landing gear, helicopter undercarriages, slat tracks and rocket motor cases – applications which require high strength-to-weight material.
Maraging steel offers an unusual combination of high tensile strength and high fracture toughness.
Most high-strength steels have low toughness, and the higher their strength the lower their toughness.
The rare combination of high strength and toughness found with maraging steel makes it well suited for safety-critical aircraft structures that require high strength and damage tolerance.

1. Maraging S teels Mr. MANICKAVASAHAM G, B.E., M.E., (Ph.D.) Assistant Professor, Department of Mechanical Engineering, Mookambigai College of Engineering, Pudukkottai-622502, Tamil Nadu, India. Email:[email protected] Dr. R.Narayanasamy , B.E., M.Tech ., M.Engg ., Ph.D., (D.Sc.) Retired Professor (HAG), Department of Production Engineering, National Institute of Technology, Tiruchirappalli-620015, Tamil Nadu, India. Email: [email protected] 1

2. Maraging steel is used in aircraft, with applications including landing gear, helicopter undercarriages, slat tracks and rocket motor cases – applications which require high strength-to-weight material. Maraging steel offers an unusual combination of high tensile strength and high fracture toughness. Most high-strength steels have low toughness, and the higher their strength the lower their toughness. The rare combination of high strength and toughness found with maraging steel makes it well suited for safety-critical aircraft structures that require high strength and damage tolerance. Introduction 2

3. 3 Cont. Maraging steel is strong, tough, low-carbon martensitic steel which contains hard precipitate particles formed by thermal ageing. The term ‘ maraging ’ is derived from the combination of the words martensite and age-hardening. Maraging steel contains an extremely low amount of carbon (0.03% maximum) and a large amount of nickel (17–19%) together with lesser amounts of cobalt (8–12%), molybdenum (3–5%), titanium (0.2–1.8%) and aluminium (0.1–0.15 %). Maraging steel is essentially free of carbon, which distinguishes it from other types of steel. The carbon content is kept very low to avoid the formation of titanium carbide ( TiC ) precipitates, which severely reduce the impact strength, ductility and toughness when present in high concentration. Because of the high alloy content, especially the cobalt addition, maraging steel is expensive.

4. 4 Cont. Maraging steel is produced by heating the steel in the austenite phase region (at about 850 °C), called austenitising , followed by slow cooling in air to form a martensitic microstructure. The slow cooling of hypoeutectic steel from the austenite phase usually results in the formation of ferrite and pearlite ; rapid cooling by quenching in water or oil is often necessary to form martensite . However, martensite forms in maraging steel upon slow cooling owing to the high nickel content which suppresses the formation of ferrite and pearlite . The martensitic microstructure in as-cooled maraging steel is soft compared with the martensite formed in plain carbon steels by quenching. However , this softness is an advantage because it results in high ductility and toughness without the need for tempering. The softness also allows maraging steel to be machined into structural components, unlike hard martensitic steels that must be tempered before machining to avoid cracking .

5. 5 Cont. After quenching, maraging steel undergoes a final stage of strengthening involving thermal ageing before being used in aircraft components. Maraging steel is heat-treated at 480–500 °C for several hours to form a fine dispersion of hard precipitates within the soft martensite matrix. The main types of precipitates are Ni 3 Mo, Ni 3 Ti, Ni 3 Al and Fe 2 Mo, which occur in a high volume fraction because of the high alloy content.  Carbide  precipitation is practically eliminated owing to the low carbon composition. Cobalt is an important alloying element in maraging steel and serves several functions. Cobalt is used to reduce the solubility limit of molybdenum and thereby increase the volume fraction of Mo-rich precipitates (e.g. Ni 3 Mo, Fe 2 Mo). Cobalt also assists in the uniform dispersion of precipitates through the martensite matrix. Cobalt accelerates the precipitation process and thereby shortens the ageing time to reach maximum hardness. Newer grades of maraging steel contain complex Ni 50 (X,Y,Z) 50  precipitates, where X, Y and Z are solute elements such as Mo, Ti and Al.

6. 6 The precipitates in maraging steel are effective at restricting the movement of dislocations, and thereby promote strengthening by the precipitation hardening process.  Figure   1  shows the effect of ageing temperature on the tensile strength and ductility of maraging steel. As with other age-hardening aerospace alloys such as the 2XXX Al, 7XXX Al, β-Ti and α/β-Ti alloys, there is an optimum temperature and heating time to achieve maximum strength in maraging steel . When age-hardened in the optimum temperature range of 480–500 °C for several hours it is possible to achieve a yield strength of around 2000  MPa while retaining good ductility and toughness . Over-ageing causes a loss in strength owing to precipitate coarsening and decomposition of the martensite with a reversion back to austenite. The strength of maraging steels is much greater than that found with most other aerospace structural materials, which combined with ductility and toughness, makes them the material of choice for heavily loaded structures that require high levels of damage tolerance and which must occupy a small space on aircraft . Cont.

7. 7 Cont. Figure 1.  Effect of ageing temperature on the strength and ductility (percentage elongation to failure) of a maraging steel.

8. 8 Metallurgy and Properties Maraging steels are low-carbon martensitic steels which employ substitutional alloying elements to achieve precipitation strengthening (age-hardening). A variety of steels have been developed which utilize this approach to achieve high strength at low carbon levels; for example, the martensitic precipitation-hardened stainless steels could be considered maraging steels. However , as Beiber’s original development of maraging compositions focused on steels containing high levels of nickel (20–25 wt%) with additions of, aluminum, titanium and niobium to impart precipitation hardening, it is customary to regard maraging steels as low-carbon precipitation-strengthened non-stainless martensitic steels ( Beiber , 1960; Decker and Floreen , 1988). 

9. 9 The discovery by Decker et al. (1962) of the effectiveness of combined additions of cobalt and molybdenum in producing precipitation strengthening in Fe–Ni martensites led to the introduction in the early 1960s of what might be regarded as the standard grades of maraging steels, designated 18NiCo(200), 18NiCo(250) and 18NiCo(300) in Table 1 (Decker and Floreen , 1988; Magnee  et al., 1974), where the 200, 250, and 300 refer to the nominal yield strengths in kips per square inch ( ksi ). There are higher-strength versions of the cobalt-containing maraging steels and some of these compositions are also given in Table 1. Representative mechanical properties of the conventional cobalt-containing maraging steels are given in Table 2 ( Magnee  et al., 1974). Cont.

10. 10 Alloy C Ni Co Mo Ti Al Mn Si 18NiCo(200) 0.03 18 8.5 3.3 0.2 0.10 0.10 0.10 18NiCo(250) 0.03 18 8 4.8 0.4 0.10 0.10 0.10 18NiCo(300) 0.03 18.5 9.0 4.8 0.7 0.10 0.10 0.10 18NiCo(350) 0.03 18 12 4.2 1.5 0.10 0.10 0.10 13NiCo(400) 0.03 13 15 10 0.2 – 0.10 0.10 18NiCo(500) 0.03 8 18 14 0.2 – 0.10 0.10 Table 1. Nominal compositions of Fe–Ni–Co maraging steels (wt%) Cont.

11. 11 Cont. Alloy Aging temperature (°C) Yield strength ( MPa ) Ultimate tensile strength ( MPa ) Fracture toughness ( MPa m 1/2 ) Charpy ( J) 18NiCo(200) 480 1316 1380 140 60 18NiCo(250) 480 1635 1690 100 35 18NiCo(300) 480 1835 1910 70 25 18NiCo(350) 480 2427 2468 40 16 13NiCo(400) 525 2530 2569 55 – Table 2. Properties of conventional Fe–Ni–Co maraging steels

12. 12 Cracking Resistance in Smooth Materials Maraging steel in the strength range 1240–1720  MPa tested as U-bends in seawater displayed good resistance, as it did not fracture in periods of up to 2–3 years although there was considerable general corrosion and fouling. However ,  microcracks  were observed after 6 months. Similar behavior 24,27  of U-bend and bent beam specimens can be expected in industrial or marine atmospheres, although general corrosion is less severe. By comparison, AISI 4340 at strength levels of 1660  MPa failed in about 1 week in both seawater and atmospheric tests. Maraging steel of yield strength at or above 2060  MPa was not resistant and failed rapidly.

13. 13 Cont. Welds in maraging steel are somewhat less resistant than base plate. U-bend exposure of 1240  MPa strength welds survived for up to 2 years in seawater, while at 1380  MPa , failures occurred in 2–18 months. 29

14. 14 It is possible to provide  cathodic protection to materials of up to 1720  MPa yield strength, by coupling to mild steel or possibly to zinc. 24,27   However , zinc and metals more active than zinc tend to induce hydrogen  embrittlement . Welds up to 1380  MPa may be cathodically protected by zinc, but at impressed potentials of −1.25 V (vs. the standard calomel electrode), both 1240 and 1380  MPa welds fail rapidly due to hydrogen embrittlement. 29   Hence , although tests on smooth specimens indicate that cathodic protection of maraging steel is possible, tests on specimens with preexisting cracks indicate a greater sensitivity to hydrogen embrittlement during cathodic polarization. 29   Cont.

15. 15 Cont. The use of cathodic protection on actual structures must therefore be applied with caution, and the application of less negative potentials than are indicated to be feasible in smooth specimen tests is to be recommended if it is assumed that structures contain crack-like defects. Further evidence of the relative resistance of maraging steel is reproduced in Figure 2. Also shown is the beneficial effect in smooth surface tests of cold rolling; shot peening  has a similar beneficial effect. 29

16. 16 Cont. Figure 2.  Bent-beam test results in aerated distilled water. These specimens were exposed to the environment at a stress of 70% of yield. Reproduced from Setterlund , R.B., 1965. Materials Protection 4 (12), 27.

17. 17  Morphologies of reverted austenite in maraging steels: (a) matrix austenite ( Schnitzer  et al., 2010b); (b) lath-like austenite (A = austenite, M = martensite ) ( Schnitzer  et al., 2010b); (c) recrystallised austenite ( Viswanathan  et al., 2005) (d) Widmanstätten austenite (Kim and Wayman , 1990). (reprinted from Materials Science and Engineering: A, 398, U.K. Viswanathan , G.K. Dey , V. Sethumadhavan , Effects of austenite reversion during overageing on the mechanical properties of 18 Ni (350) maraging steel, 367–372, 2005, with permission from Elsevier); (reprinted from Materials Science and Engineering: A, 128, Sung- Joon Kim, C. Marvin Wayman , Precipitation behavior and microstructural changes in maraging Fe-Ni- Mn -Ti alloys, 217–230, 1990, with permission from Elsevier)

18. 18 Microstructure of 18Ni (250) Maraging steel, a) lath martensite after forging-light microscopy, b) lath martensite -SEM, c) lath martensite -TEM, d) Ti(N,C) at austenite grain boundary-TEM 

19. 19 Cont. Overview of high strength steel concepts ( Scripta Materialia 60 (2009) 1141)

20. 20 Applications of Maraging Steel Production tools Aerospace and aircraft parts Aluminium and zinc dies Casting and forging dies Carbide die holders Extrusion press rams, dies, and containers Gears for machine tools Index plates Pistons Springs Stub shafts Cold reducing mandrels Anchor rails Arresting hooks Gimbal ring pivots Rocket motor cases Load cells Shock absorbers for lunar rover Universal flexures Military Others Cannon recoil springs Lightweight portable military bridges Rocket motor cases   Auto-racing car parts (rods, shafts, gears) Uranium enrichment plants parts (rotors, shafts) Cable sockets Hydraulic hoses Pump impellers and casings Tensile test equipment Rotors for ultracentrifuges Different application areas of maraging steels. 

21. 21 Cont.

22. 22 Maraging steels are low-carbon martensitic steels which employ substitutional alloying elements to achieve precipitation strengthening (age-hardening ). From: Reference Module in Materials Science and Materials Engineering, 2018 References: Warren M. Garrison, Malay K. Banerjee , Martensitic Non-Stainless Steels: High Strength and High Alloy☆, Reference Module in Materials Science and Materials Engineering, Elsevier, 2018, Thank You Authors of Technical articles and Scopus Journals are Acknowledged.