Microstructure,properties and hot workability of M300 grade maraging steel

K.V.A.Chakravarthi,N.T.B.N.Kouninya,S.V.S.Narayana Murty,B.Nageswara Rao

aMaterials and Mechanical Entity,Vikram Sarabhai Space Centre,Trivandrum 695 022,India

bResearch scholar,Department of Mechanical Engineering,K L University,Green Fields,Vaddeswaram,Guntur 522 502,India

cDepartment of Mechanical Engineering,K L University,Green Fields,Vaddeswaram,Guntur 522 502,India

dDepartment of Metallurgical Engineering and Materials Science,Indian Institute of Technology,Madras,Chennai 600 036,India

1.Introduction

Maraging refers to the ageing of martensite.Maraging steels are primarily based on the Fe-Ni system having very costly and strategic Ni and Co as alloying elements.The ultra-high strength 18%Ni maraging steels are attracted by material scientists and structural designers of aerospace,nuclear and defence industries[1-5].They are classified into M200,M250,M300 and M350 grades according to their 0.2%proof stress or yield strength levels,namely 200,250,300 and 350 ksi.They are still of interest for investigations[6-16].

Thermo-mechanical processing,an important step in the manufacturing,is used to obtain the desired shape and microstructural changes.Process control parameters such as temperature and strain rate are to be selected to avoid flow localization or cracking during hot deformation.Isothermal compression testing of solid cylindrical samples is widely used for the microstructural evolution and hot workability evaluation in the laboratory to select process parameters and determine the material constants in the constitutive relations.However,the actual conditions during thermo-mechanical processing on shop floor are non-isothermal with the dies at lower temperature than that of the workpiece.Utilizing the flow stress data and performing numerical simulations on the workpiece,it is possible to estimate the strain,strain rate and temperature in non-isothermal conditions.

This paper presents the results of isothermal uniaxial compression tests on M300 grade maraging steel useful in nuclear and aerospace industries.Material constants in a constitutive relation are evaluated.Processing map is generated and identified the optimum processing parameters to obtain defect free products.

2.Flow stress data generation

As in Ref.15,processing of the M300 grade maraging steel is done by vacuum induction melting followed by vacuum arc remelting.The as-cast billets are of size φ580 mm and 1500 mm long.The as-cast billets were homogenized for 24 h and conducted two intermittent primary deformation operations at 1200°C.Later on,the billets are forged to φ45 mm(square cross-section),process annealed and ground to remove scales formed during hot deformation.The chemical composition of the material was(%wt):0.003C-0.01Mn-0.02Si-0.002S-0.004P-Cu-0.02-0.03Cr-9.0Co-18.4Ni-4.94Mo-0.71Ti-0.12Al-Balance Fe.In the annealed condition,the ultimate tensile strength,σult=1065MPa at room temperature.In the aged condition(482°C for 6 h followed by air cooling),the achieved properties were:ultimate tensile strength,σult=2058MPa;0.2%proof stress or yield strength,σys=2005MPa;%elongation=10;%reduction in area=53;and hardness=54HRC.

Isothermal compression tests were conducted on cylindrical specimens of 10 mm diameter and 15 mm height at temperatures of 900,950,1000,1050,1100,1150 and 1200°C and at strain rates of 0.001,0.01,0.1,1.0,10 and 100s-1.Samples were compressed up to the strain of 0.5 and subsequently water quenched to freeze the microstructures evolved.Fig.1(a)shows the schematic of the test plan and Fig.1(b)shows the photograph of the compressed samples having symmetric deformation and the absence of surface cracks.Microstructures were recorded from the polished and etched sample observations under Olympus-GX71 inverted metallurgical microscope.

Electron back scatter diffraction(EBSD)analysis has been carried out on selected samples in an EDAX™EBSD-OIM(electron backscattered diffraction-orientation imaging microscopy)system attached to a FEI™NOVA NanoSEM FE-SEM(field emission scanning electron microscope).The samples were electropolished using a solution of 80%methanol and 20%perchloric acid at 5°C.

3.Results&discussion

Investigations were made on microstructure,properties and hot deformability of M300 grade maraging steel.The interfacial friction had negligible effect on the flow stress values as in Ref.15.Fig.2 shows the stress-strain curvesgenerated from isothermal compression testing in the temperature range of 900-1200°C and in the strain rate range of 0.001-100 s-1.The flow stress decreases with increasing temperature and decreasing strain rate.Based on the microscopic mechanism during plastic deformation,the stress-strain curve can be divided into work hardening region(Stage I),transition region(Stage II),softening region(Stage III)and steady state region(Stage IV).Initial strain hardening is observed in the temperature range of 900-1000°C and in strain rate range of 0.1-100 s-1.No strain hardening in the flow curves is seen for the specimens deformed at 1150 and 1200°C.The microstructure is austenitic and exhibits low stacking fault energy(SFE)during deformation in the range of 900-1200°C.

Fig.3 presents the initial microstructure of the material showing lath martensitic structure in the annealed condition.Fig.4 shows optical microstructures of the deformed specimens.The deformed microstructures are replaced gradually by recrystallized microstructures with increasing temperature at a given strain rate.Microstructures reveal reconstitution at low temperatures and strain rates,whereas deformed microstructures were observed at high strain rates.Grain growth is noticed at high temperatures and low strain rates.Deformation at high strain rates results in a fully recrystallised microstructure.Microstructures reveal reconstitution at the low strain rate of 0.001s-1and temperature of 900°C,and also at high strain rate of 100s-1and temperature of 1050°C.

Processing maps are in general are obtained from the flow stress-strain curves generated at different temperatures and strain rates.Such maps with the axes of temperature(T)as abscissa and strain rate(έ)as ordinate show the stable(safe)and unstable(unsafe)domains that are likely to occur during hot deformation of materials[17,18].They are representation of material behavior in terms of its microstructural evolution under varying strain(ε),strain rate(έ)and temperature(T).They consist of power dissipation map,strain rate sensitivity contour map and an instability map.Instabilities in material flow(like localized flow,adiabatic shear or cracking)are observed,if the rate of entropy production in an alloy system does not constitutively match with the rate of entropy input by imposed process conditions.Murty et al.[19]have examined various instability theories for identifying the temperature-strain rate domains of flow instabilities during hot deformation of materials.The dynamic material model(DMM)introduced by Prasad et al.[20]considers the workpiece as a dissipater of power.

The total power dissipated by the work piece is

The first integral(G)in equation(1)is the power input dissipated as temperature rise whereas the second integral(J)gives the power dissipated by metallurgical processes and its relationship withGis given by

wheremis the strain rate sensitivity parameter.

The efficiency of power dissipation （η）is defined as[21].

The condition for the metallurgical instability is given by Ref.[22].

Negative values of instability parameter ξ（˙ε）indicate that the material flow is unstable in that region.Contour maps of the efficiency of power dissipation(η)and the strain rate sensitivity parameter(m)at the strain,ε=0.5 for M300 maraging steel are presented in Fig.5(a)and Fig.5(b)in which the numbers represent the values of efficiency of power dissipation and the strain rate sensitivity parameter respectively.The peaks ofmand η signify deformation mechanisms.Equation(4)is utilized to develop the instability map for M300 maraging steel in the temperature range of 900-1200°C and strain rate range of 10-3to 102s-1.The instability map generated at ε=0.5 in Fig.6 exhibits three distinct domains as follows.

The range of η in Domain I is 0.10-0.40,Domain II is 0.40-0.60 and Domain III is 0.20-0.40.The peak value of η in efficiency map occurs in Domain II at high temperature and at strain rate of 10-3s-1,whereas the maximum value ofmin this domain is 0.35.The low efficiency is observed in the domain of low temperature and high strain rate.The rate of microstructural restoration processes and also the mobility of grain boundaries of recrystallised grains,increase at high temperature,whereas the rate of nucleation and growth of new grains improve at low strain rates.The highest power dissipation efficiency in the DRX region is 60%occurring at 1175°C and 10-2s-1.The microstructural instability in Fig.6 occurs in the strain rate range of 10-1to102s-1over the temperature range of 900-1000°C;10-1to 10°s-1over temperature range of 1000-1200°C;and 101.5to 102s-1over the temperature range of 950-1200°C.

The optical microstructures of recrystallised samples have inherent difficulty in revealing the packet boundaries and cannot provide finer aspects of recrystallised microstructure.Hence EBSD plots are used to analyze the microstructural features of hot compressed samples.Fig.7 shows inverse pole figures and image quality maps with superimposed boundary maps of samples deformed at 1200°C and strain rates of(a,e)0.01s-1(b,f)0.1s-1(c,g)10 s-1and(d,h)100 s-1.Red lines represent low angle boundaries with misorientation(2°-5°),green lines represent medium angle boundaries with misorientation(5°-15°)and blue lines represent high angle grain boundaries(＞15°).These maps show random orientation of martensite laths indicating complete reconstitution of the deformed microstructure during deformation.Table 1 presents the fraction of different types of boundaries in M300 grade maraging steel samples deformed at 1200°C and different strain rates.For a low strain rate of 0.01s-1,significant fraction(70%)of low angle boundaries is seen within the coarse martensite laths.For the strain rate of 0.1 and 10s-1,the fraction of medium angle boundaries increased significantly.The martensite packet size is decreased with increasing strain rate.Although increase in the fraction of high angle boundaries with increasing strain rate is noticed from 20 to 37s-1,significant portion of low angle boundaries are also noticed at the highest strain rate of 100s-1.These observations are supported by the misorientation and cumulative misorientation plots of the deformed samples(see Fig.8).These observations are interesting from the point of view of increasing productivity as the fraction of high angle boundaries is found to be higher at higher strain rates suggesting some extent of DRX even at intermediate strain rates(0.1 and 10s-1).

Fig.9 shows the variation of Kernel Average Misorientation(KAM)for the samples deformed at different strain rates at 1200°C.The fraction of low KAM increases(suggesting increase in dislocation density)with increasing strain rate with the exception at 100s-1.Dynamic recovery(DRV)is dominant at high strain rates(except for 100s-1)whereas dynamic recrystallisation(DRX)is dominant at high temperatures and low strain rates.Though DRV is dominant at high strain rates,the deformation at 1200°C causes some extent of DRX even at strain rates of 0.1 and 10s-1resulting high fraction of medium angle boundaries(Fig.8(b)).

The safe processing window for M300 maraging steel is in the temperature range of 1125-1200°C and strain rate range of 0.001-0.1s-1.EBSD analysis(Fig.7(f)-(g),8(b),(9))indicates the processing window in the temperature range of 1050-1200°C and strain rate range of 1-32s-1useful for productivity.

Following Lin et al.[23,24],a relation between the flow stress（σ）,strain（ε）,strain rate（˙ε）and temperature(T)can be written from the generated flow stress data of M300 grade maraging steel in the form

Table 1Fraction of different types of boundaries in M300 grade maraging steel samples deformed at 1200°C and strain rates.

Fig.10 shows a good comparison of experimental flow stress values with those obtained from equation(6)within the specified temperature range of 900-1200°C,strain rate range of 0.001-100 s-1and strain range of 0.1-0.5.

4.Conclusions

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