Fe-Ni-Mn Maraging Steels: Part Two
Maraging steels are a special class of low‐carbon ultra‐high‐strength steels which are produced by precisely controlling the amounts of nickel and manganese in the finished material.
The maraging response is highly dependent on the manganese content and at precise levels can produce profound hardness visible through classical age hardening curves as a function of aging temperature and time.
It is well known that Fe-Ni-Mn alloys show remarkable age-hardening behavior in the temperature range from 300 to 500°C. Although age hardening in these alloys has been studied extensively, the identity of precipitate particles in Fe-Ni-Mn alloys remains controversial. The f.c.t. θ-NiMn, b.c.c, β-NiMn, f.c.c. Ni3Mn and an ordered b.c.c. precipitate have been reported. These diversified opinions may be due to a variety of alloy compositions, various heat treatments or uncertainties of identification in electron diffraction analyses. The most common technique has been based on diffraction patterns from extracted precipitate particles. However, there are problems inherent in extraction methods in that lines from inclusions and oxides formed by the extraction solution may obscure the results. It should be noted that all reported results do not provide substantive experimental evidence for the specific precipitates.
In the study of Sung-Joon Kim and C. M. Wayman, the crystallography, the orientation relationship with the matrix and the shape of precipitates have been investigated using thin foil analytical electron microscopy and computer simulation of selected-area diffraction patterns (SADPs) for Fe-20wt.%Ni-Mn alloys.
The alloy compositions of the Fe-Ni-Mn studied are Fe-21.1wt.%Ni-1.27wt.%Mn (alloy A-0) and and Fe-20.8wt.%Ni-3.23wt.%Mn (alloy B-0); alloy A-0 has a lower manganese content than B-0 has.
The maraging response of the alloys investigated in this study was strongly dependent on the manganese content. The higher manganese alloys showed a similar precipitation behavior to that in low nickel Fe-Ni-Mn alloys with classical age hardening curves as a function of aging temperature and time. Fine spherical precipitates appeared first at 400°C aging, followed by the appearance of rod-shaped f.c.t. θ-NiMn and Widmanstätten austenite. Ordered f.c.t. θ-NiMn precipitates were identified using EDX analysis and confirmed by computer simulation of SADPs. The pronounced hardening is associated with the stress required for dislocations to cut through coherent zones or precipitates of spherical precipitates. The orientation relationship between metallic parent matrix and θ-NiMn precipitates is [011]M//[111]θ and (ĺ ĺ 1)M//(ĺ01)θ, whereas that between the martensite and Widmanstiitten austenite is the Kurdyumov-Sachs relation. The shapes of the θ-NiMn precipitates are dependent on the aging temperature. Rod-shaped θ-NiMn 16 precipitates become dominant at lower aging temperatures, while disc-shaped θ-NiMn appears at higher aging temperatures.
Floreen and Decker have shown that in 18 pct Ni maraging steels, the early stages of hardening may be represented by the empirical relationship:
Ht – H0 = ΔH = (Kt)n
where Ht = hardness at time t
H0 = initial hardness
K = constant
n = constant
During investigation of an Fe-12Ni-6Mn maraging alloy a similar behavior was found to occur (see Figure 1). This alloy consists of lath martensite in the air cooled condition and shows pronounced hardening on aging in the temperature range 350°C to 500°C (Figure 2). Hardening is due to precipitation of θ NiMn on dislocations (Figure 3).
Figure 1: Variation of initial hardness increase ΔH with aging time, t, in an Fe-12Ni-6Mn maraging alloy
Figure 2: Age-hardening curves for alloy
Figure 3: Thin foil electron micrograph of overaged alloy