Microstructural Analysis of Co-Free Maraging Steel Aged

ABSTRACT: Maraging steels have low carbon content and are highly alloyed, having as main feature the ability to increase the mechanical strength after thermal aging. Therefore, the objective of this work is to analyze the effect of aging in a Cofree maraging steel, Fe – 0.014% C – 0.3% Mn – 3.9% Mo – 2.1% Cu – 0.19% Si – 11.8% Cr – 9.1% Ni – 1.0% Ti (wt), at different heat treatment times (10 min 960 min) at constant temperature (550°C), after cold rolling up to 66% and 77% area reduction. Microstructures were analyzed by scanning electron microscopy (SEM EDS) and mechanical properties by Vickers hardness measurements. The results showed a significant increase in the hardness of the material after aging heat treatment on the solution treated and deformed samples. The aging heat treatment which was harder about 650 HV was 550°C/60 min.

Steel with martensitic structure, when subjected to a heat treatment at a constant temperature, suffers a growth of intermetallic precipitates uniformly in the matrix (Carvalho et al., 2013).The precipitate size is a contributing factor to the increased mechanical resistance of maraging steels.The smaller the precipitates and the more coherent with the matrix, the greater the hardness of the alloy, since there is less space between the particles; furthermore, they act as barriers to movement of dislocations (Padial et al., 2000).
The alloying elements of maraging steel, such as molybdenum, titanium, nickel, cobalt, aluminum, and others, have a great influence on the mechanical properties of this alloy, in other words, they are responsible by precipitates hardeners (Carvalho et al., 2013).However, recent studies (Hu et al., 2008;Leitner et al., 2011;Mahmoudi et al., 2011;Mahmudi et al., 2011;Nili-Ahmadabadi, 2008;Schnitzer et al., 2010;Sha et al., 2013) are being conducted in maraging steels without cobalt addition, in order to reduce the production costs of this alloy.Maraging steels are considered expensive due to their materials' preparation and expensive alloy elements such as nickel and cobalt processes.The elimination of the cobalt content and the substitution of nickel by cheaper elements, such as manganese, has been studied.Cobalt is used only to minimize the solubility of molybdenum.In the case of maraging steel studied, this element was replaced by chrome, which besides improving the hardenability increases the corrosion resistance (Nili-Ahmadabadi, 2008).
Maraging steels are mainly applied in the aeronautical, aerospace, nuclear and military industries, as they have great advantages such as good weldability, high strength, high yield strength, high fracture toughness, they support high working temperatures, have good machinability, good formability, among others, with great applicability matrices and tools (Lopes, 2007).
High-tensile steels such as 300M and SAE 4340 have extensive application in aeronautics and aerospace industry, being mainly applied in the manufacture of structural components (Zang et al., 2013;Boakye-Yiadom et al., 2014).
Maraging steels are being investigated because of the possibility of replacing the commercial alloy steels, 300M and 4340 for example, widely used in aeronautical industry (Carvalho et al., 2013).Thus, this study aims to examine the effect of aging heat treatment in non-commercial maraging steel (Co-free), in order to compare it to the 300M and 4340 steels.

EXPERIMENTAL
The maraging steel used in this work was produced by vacuum induction melting.The ingots was hot forged and then hot rolled until we obtained bars of 20x20 mm of dimension and chemical composition: Fe -0.014% C -0.3% Mn -3.9% Mo -2.1% Cu -0.19% Si -11.8% Cr -9.1% Ni -1.0%Ti (wt.%).
Its bar (20x20 mm) was separated into two parts resulting in two conditions: • Condition I (C-I): bar dimension of 20 x 20 x 10 mm; • Condition II (C-II): bars of 20 x 20 x 10 mm (~100 wt.), remelted (1 remelt) on an arc furnace, resulting in an ingot dimension of 16x10x95 mm.
Both conditions (C-I and C-II) were solution treated at 1050°C during 1 hour, quenched in water at room temperature and then cold rolled to a plate of 3 mm in thickness, resulting in 66 and 77 % of area reduction (AR) for C-I and C-II, respectively.The deformed samples (AR = 66 and 77 %) were cut in several parts and each part was annealed at 550°C for different times, ranging from 10 to 960 min.
Microstructural characterization was performed using a Scanning Electron Microscope (SEM) VEGA 3 TESCAN model, equipped with an energy dispersive spectrometer system (EDS).Vickers hardness testing was performed using a Future-Tech FM-7000 microindenter with a load of 200 g/15s, in which the results correspond to the mean of ten values taken on each specimen.The micrograph analyses and the Vickers' hardness were undertaken in a longitudinal section of the samples (wire rolling direction).

RESULTS AND DISCUSSION
Figure 1 shows the hardness variation as a function of heat treatment time for maraging steel with AR's = 66 and 77 %.For solution treated (C-I and C-II) and deformed samples, the hardness values are represented at zero time in Fig. 1.
Solution treated samples (C-I and C-II) showed average hardness values of 250 and 300 HV, respectively.Comparing the solution treated to the deformed condition, it is observed an increase around 50 % in average hardness values (HV 66% = 425 and HV 77% = 407), corresponding to the effect of work hardening.About thermal aging, there is a significant increase in hardness at relatively low heat treating times (Fig. 1).For both area reductions, increased hardness around 200 HV occurs with only 10 min thermal aging.He et al. (2002) observed that the heat treatment at 470°C performed in a maraging steel with 18%wt.Ni, after forging, has achieved a 90% increase in hardness, with only 15 min of aging.
In the detail of Fig. 1, it is observed that the maximum hardness is obtained at 550°C/60 min for both ARs.On the other hand, the thermal aging at 550°C exhibits a decrease in hardness values after 240 min for deformed sample with AR = 66%, whereas a decrease it is observed after 90 min to condition AR = 77%.Therefore, it can be said that in smaller ARs, there is an increase in aging time of the alloy, delaying the onset of overaging.Based on the average hardness values, the processing) in the matrix, resulting in a lath martensitic structure (Mahmoudi et al., 2011;Hu and Wang, 2012), as shown in Figs.2a and 2b.Co-free maraging steel is stable up to 16 hours of aging at 550°C.    Figure 2 shows the microstructure of the maraging steel at different conditions.It is observed for solution treated sample (C-I and C-II) that the solution heat treatment at 1050°C for 1h was effective in dissolving primary precipitates (formed during the early stages of maraging steel

Hardness (HV) Time (min)
After thermal aging at 550°C for 1 hour (Figs.2e and 2f), it can be seen that the microstructure is similar to deformed samples in AR = 66 and 77 % (Figs.2c and 2d).This heat treatment time showed to be effective in relation to increased hardness (Fig. 1), due to presence of fine precipitates dispersed in the matrix, which contribute to the increase of this mechanical property (Hu and Wang, 2012).However, the technique of scanning electron microscopy, used in order to analyze the microstructure of this alloy, is not efficient to identify them in the matrix.
Increasing the annealing time to 16 h (Figures 2g and 2h) promoted an increment in precipitation rate.These precipitates grew coarser in subsequent higher annealing times, but there still are fine precipitates elongated in the rolling direction, and few precipitates changed from elongate morphology to a spherical one.In this condition, it is observed some coalescence of precipitates having sizes of the order of 2 μm.Regardless of AR, the precipitates have similar morphologies.The EDS analysis (Table 2) showed that those precipitates, referred as P2 and P5, were depleted in Ni, and enriched in Mo and Ti (Figures 2g and 2h).
In the analysis performed on the matrix -points 1 and 4 in Figs.2g and 2h, respectively -, compared to the nominal composition of the alloy, it can be said that the rolling processes or thermal treatments did not significantly alter the composition of the material in terms of Mo, Ti, Cr and Ni elements.
In relation to precipitates P2 and P5, a large increase of Mo and Ti was observed.This alloy contains low nickel, so it is not possible to stimulate the formation of Ni 3 Mo or Ni 3 Ti phases, which usually appear in the commercial maraging steels 250 and 350 (Padial et al., 2000).The presence of cobalt element in commercial alloys promotes the formation of Ni 3 Mo.On the other hand, the maraging steel study in this work does not have Co (Co-free) and low Ni content.However, the alloy phase formed in this alloy can be Fe 2 (Mo, Ti), FeMo, FeTi or Fe 7 Mo 4 (Padial et al., 2000).The precipitate P3 features a larger enrichment of Ti in relation to P2 and P5, probably, the Ti-rich precipitates are carbides (TiC) or Fe 2 Ti, FeTi (Castanheira et al., 2006).The TiC is formed by carbon, which is characterized as an impurity of the material (Padial et al., 2000).Furthermore, other techniques are necessary, for example, an X-ray diffraction to determine the type of precipitate formed.
The SAE 4340 and 300M steels are considered hightensile steels, low-carbon and low-alloy steels, being used in the aerospace industry due to their excellent mechanical properties.Commercial maraging steels are being studied in order to replace the 4340 and 300M steels, especially in the aeronautical and aerospace industry.According to studies by Cardoso et al. (2013b), the SAE 4340 and 300M steels can achieve hardness values of 250 HV and 350 HV, respectively, after drawing back.However, the maraging steel studied with forming processes and appropriate heat treatment can achieve hardness values around 670 HV, i.e., almost double the amount reported by the 300M steel.

CONCLUSION
Based on this investigation, the following conclusions can be drawn: • The cold forming contributes to mechanical hardening, however an effective increase of hardness is obtained after thermal aging; • The hardness increased significantly with low time thermal aging at 550°C due to precipitation of the second phase particles.The maximum value obtained was of 650 HV at 550°C/60 min; • Based on the average hardness values, the Co-free maraging steel is stable for up to 16 hours of aging at