Short-term Creep Properties and Fracture Surface of 18 Ni ( 300 ) Maraging Steel Plasma Nitrided

Plasma nitriding of a solution annealed and aged 300 grade maraging steel was studied aiming to increase the creep resistance. The surface microhardness reached 1,140 HV, producing 50 μm layer composed of ε-Fe3N and γ’-Fe4N nitrides at the uppermost sample layer. The inner core remained unaltered presenting typical plate-like martensite microstructure of maraging steels with average microhardness of 604 HV. Surface RMS roughness in the nanometric scale increase from 52 nm to 71 nm. The continuous layer of iron nitrides seems to behave as a barrier for oxidation and for inward oxygen diffusion improving the creep resistance by reducing the steady-state creep rate (εs) in 52-65% when compared with the literature results. Dominant creep mechanism is controlled by dislocations climb. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45o shear lip. Nitrided sample presented a reduced ductility, associated to the hard surface layer.


Introduction
The 18-percent nickel 300 grade maraging steels belong to a family of iron based alloys that are strengthened by a combination of martensite formation during a solution annealing followed by an aging treatment.Since 1959 when their development was announced, maraging steels evoked tremendous interest, especially in the aerospace world, due to the combination of structural strength and fracture toughness in a material that was, at the same time, readily weldable and easy to heat-treat.The large rocket-motor program sparked the considerable research and development of maraging steels, and it is the key application until now.Otherwise, currently maraging steels are applied not only for aerospace, military and nuclear industries, but also for transportation, manufacturing, tooling, die making and electromechanical components.300 grade maraging steel is a member of ironnickel based alloy family with yield strength of 300 ksi [1][2][3][4] .
One of the major factors limiting the life of maraging steels in service is their degradation due to gaseous environments, in particular, to environments containing oxygen at elevated temperatures during long-term use.When maraging steels are exposed in air, obey a limited oxide layer thickness growth law up to 480 ºC and a parabolic law above this temperature.At 500°C and up to 600°C, the growth rate is faster and increases sharply with increase in temperature.The interaction of maraging steels with oxygen not only causes losses in the material during the formation of oxides but also causes embrittlement in the subsurface zone of the component due to oxygen enrichment, and the creep resistance is often the limiting parameter in design under different temperaturestress domains [5][6] .Therefore, for high temperature service in oxidizing atmospheres, satisfactory creep resistance must be combined with adequate resistance to environmental degradation.Increasing efforts are being directed to the study of superficial thermochemical treatments to improve oxidation and creep resistance of metals and alloys [7][8][9][10] .Plasma nitriding is a technique used to introduce elemental nitrogen to the surface of a metal part for subsequent diffusion into the material by ion glow discharge.Plasma nitriding is specially designed for low temperature nitriding, and it is adequate for maraging steels avoiding the overaging [11][12][13] .Plasma nitriding of maraging steels has been extensively investigated with the aim of improving the alloy's tribological and mechanical properties [13][14] .Otherwise, data available on the mechanical   , 15-18 .In this study, the creep resistance and fracture surface of a plasma nitrided maraging 300 steel is evaluated and compared with the results obtained by Reis et al. 18 for the same material but without surface treatment.

Experimental Procedures
In this study it was used a 300 grade maraging steel with the chemical composition according to Table 1.The samples were solution treated at 820 ºC -1 h and then air cooled followed by aging at 480 ºC -3 h and then air cooled in a Brasimet Koe 40/25/65 furnace, hereafter called MAR-SA.Plasma nitriding was performed using a 30 kW DC-pulsed source at 480 ºC for 3 h in an environment atmosphere of 75% N 2 -25% H 2 , hereafter called MAR-SAP.The microstructural characterization was carried out on Carl Zeiss model Axio Imager 2 optical microscopy -OM.XRD was carried out at room temperature with a CuKα radiation source on Panalytical model X´ Pert Powder diffractometer.Data for each phase were analyzed with X'Pert HighScore using the ICDD database.Hardness was measured using Vickers microhardness Tester (FutureTech model FM-700) with load of 100 g for 9 s.Surface morphology was analyzed using Nanosurf Flex atomic force microscope -AFM (tapping mode, SiN 3 tip model NCHR, Nanoworld).Samples of 18.5 mm gauge length by 3.0 mm in diameter were submitted to constant load creep tests at 650 ºC (200, 300 and 500 MPa), 600 ºC (500 MPa) and 550 ºC (500 MPa) in a standard Mayes creep machine, according to ASTM E139 standard 19 .Disks for transmission electron microscopy (TEM) were sliced from the crept samples and pre-thinned by dimple grinder SBT Model 515.Thin foils were then prepared by ion polishing in a GATAN Model 691.Observations of the center of the foils were carried out in a FEI TECNAI model G2F20 TEM.Fractographic analysis were carried out by scanning electron microscopy -SEM (TESCAN model VEGA 3).The X-ray diffraction (XRD) pattern in the 2θ ranging from 35-90º of MAR-SA and MAR-SAP samples are shown in Figure 2. MAR-SA exhibit diffraction peaks only due to the martensitic phase α'-Fe, Figure 2a.After plasma nitriding (MAR-SAP), α'-Fe peaks disappeared or are overlapped, giving place to peaks indexed as ε-Fe 3 N (01-076-0091) and γ'-Fe 4 N (00-001-1219) nitrides, corresponding to the compound layer, Figure 2b.

Microstructural characterization
The surface microhardness increased from 604 ± 18 HV (MAR-SA) 18 to 1,140 ± 15 HV (MAR-SAP), after plasma nitriding.The surface microhardness increase of 88.7% is associated to the formation of ε-Fe 3 N and γ'-Fe 4 N nitrides, revealed by the XRD result from Figure 2b. Figure 3 shows the microhardness profile of MAR-SAP.A comparison of the depth etched out in Figure 1 and the depth-hardness variation in Figure 3 reveals that the etched-out depth (50 μm) is the depth of the layer where the nitride phase is predominant, as proved in X-ray diffraction in the Figure 2. Higher values of microhardness are expected in this region.The band from 50 to 100 μm is the transition region where the hardness values decrease inward in a different rate, approaching the hardness of the matrix.This is probably the region of solid solution hardening by nitrogen atoms.It should be noted that the core hardness of MAR-SAP remains the same as that of MAR-SA.This confirms that the core does not soften by overaging due to ion nitriding temperature and time.
Plasma nitriding changed the topography on the surface of MAR-SAP sample, according to the three-dimensional AFM images (Figure 4).The layer of iron nitrides being formed led to an increase of surface RMS roughness in the nanometric scale from 52 nm (MAR-SA), Figure 4a, to 71 nm (MAR-SAP), Figure 4b, as measured by AFM microscope.The nitrided surface presented a typical flakelike peaks morphology.

Creep behavior
MAR-SAP samples exhibit typical creep curves consisting of well-defined primary (I), secondary (II) and tertiary (III) stages.Figure 5 displays representative creep curves of strain (ε) versus time (t) at 650 °C and 200 MPa.The results from creep tests are summarized in Table 2, which shows the values of the steady-state creep rate (ε̇s), the time to rupture (t r ) and the percent of elongation (EL).For comparison, the creep data of MAR-SA 18 is also available in Figure 5 and Table 2.The ε̇s values of MAR-SAP are 52-65% lower and the t r are 20-33% higher, when compared to MAR-SA.The reduction of ε̇s and increase of t r indicate that a higher creep resistance of MAR-SAP plasma nitrided samples.This fact is related to surface microstructure formed in the plasma nitriding.The continuous layer of ε-Fe 3 N and γ'-Fe 4 N nitrides behave as a diffusion barrier for inward oxygen diffusion (into the alloy), reducing the oxidation rate and improving the creep resistance of the steel.The creep results of maraging 300 steel (nitrided an unnitrided) at the temperature range of 550 °C to 650 °C is also affected by martensite reversion to austenite, that deteriorates mechanical properties.The literature reports an A s (Austenite formation start) of 623 ºC and A f (Austenite formation finish) of 801 ºC, but exposure at temperatures even below A s can produces different amount of reverted austenite.The reason is that the martensite that is formed during solution treatment is metastable and the system decomposes to the equilibrium austenite and ferrite structures via diffusioncontrolled reactions 18,[20][21][22] .Specialized applications of the steel occasionally demand short-time exposures to high temperatures and it is desirable to have data on the creep behavior of the material during such service conditions.
For most metal and alloys, the relationship between the steady-state creep rate (ε̇s), stress (σ) and temperature (T) can be expressed by the power-law creep equation: where Q c is the activation energy for creep, A is a constant that depends on the microstructure, temperature and applied stress (σ), n is the stress exponent, R is the universal gas constant and T absolute temperature.The combination of Q c and n values indicates the main creep mechanism that controls a given deformation process 23 .The slopes of the plot lnε̇s x lnσ, which is presented in Figure 6, provide an estimate of n.The dependence of the steady-state creep rate on temperature at 500 MPa is presented in Figure 7 and Table 3.The values of n = 7.1 / Q c = 428 kJ/mol (MAR-SA) 18 and n = 6.8 / Q c = 388 kJ/mol (MAR-SAP) are in accordance with results reported in the literature to materials hardened by a dispersion of a second-phase particles that are considerably higher than the values expected for the appropriate base materials (iron: n = 5 and Q c = 284 kJ/mol).The expected stress exponent values for these materials are in the range of 5 to 15 23,24 .Furthermore, Viswanathan et al. 15 who used indentation technique for evaluating high temperature creep of 350 grade maraging steel, obtained Q c values between 405 kJ/mol and 467 kJ/mol.
The analysis of the values of the activation energy and stress exponent suggests that the creep mechanism is associated with dislocation climbing process 23,24 .Figure 8 shows TEM image of the crept samples at 650 ºC and 500 MPa.The micrograph depicts dislocation cell structures.

Characteristics of fracture surfaces
Both the MAR-SA 18 and the MAR-SAP samples showed a cup-and-cone fracture morphology after creep tests, characteristic of a ductile type of failure.Figure 9a is a representative SEM micrograph showing the general view of the fracture surface of the MAR-SA sample crept at 650 ºC and 300 MPa, consisting of dimpled rupture surrounded by shear lips.Figure 9b is a higher magnification image of the equiaxed dimples at the location enclosed in Figure 9a.
Otherwise, from Table 2 it can be noticed that the ductility (elongation, %) decreased after plasma nitriding, considering all stress conditions of creep tests.The ductility of the MAR-SAP are 17 to 40% lower than that of MAR-SA samples.Ductility reduction of nitrided sample is also observed by fractographic analysis.
The fracture surface of MAR-SAP crept at 650 ºC and 300 MPa is illustrated in Figure 10a, showing a dimpled rupture region at the center surrounded by shear lips and encircled by a narrow rim.Equiaxed dimples were observed in the dimpled region in the center part of fracture surface, (Figure 10b).Higher magnification observation revealed that the narrow rims presented a flat fracture with cleavage facets, (Figure 10c).The average width of the narrow rim is 100 μm, and it is close to the value found in the microhardness   profile (Figure 3).It can be associated to the iron nitrides formed and nitrogen solution hardening regions after plasma nitriding.Microcracks initiating from the surface were observed near the surface as shown in Figure 10d.The higher ductility of MAR-SA compared to MAR-SAP could also be inferred from top view of Fig 9a and 10a respectively.The fibrous fracture area of MAR-SA is 35% smaller than that of MAR-SAP indicating higher ductility for the first.
Figure 11 is a side view SEM image of MAR-SAP sample after rupture showing the formation of circumferential (transverse) microcracks on the nitrided surface perpendicular and cross-linked at 45º angle relative to the tensile loading direction.
Creep fracture has been described by the phenomenological Monkman-Grant relationship 25 , which states that the fracture of creep-deforming material is controlled by the steady-state creep rate according Equation 2: Where M is a constant typically about 1.0 (when following the Monkman-Grant relationship) and C is referred to as Monkman-Grant constant.However the value of C is material dependent, the values normally observed is in the range of 0.001 to 0.1.As a general rule, larger C values tend to be recorded for materials displaying higher creep ductility 23 .Figure 12 shows the Monkman-Grant correlation of MAR-SA and MAR-SAP.For both cases, M is around 1.0 (MAR-SA M = 1.017 and MAR-SAP M = 0.968), independent of creep testing temperature and stress regime.This indicates that the creep rupture of the 300 grade maraging steel nitrided and un-nitrided follow the Monkman-Grant relationship for Therefore, ductility measurement, fractography and Monkman-Grant relationship show a reduction of ductility and cracks propagation in the surface in the MAR-SAP when compared to MAR-SA 18 .During the plasma nitriding, nitrogen atoms are diffused into the surface and near-surface regions.Because the atoms occupy space in the lattice, they tend to produce compressive residual stress parallel to the surface that can be used to neutralize, or counteract, potentially damage tensile applied stress 26 .Therefore, the compressive residual stress on the nitrided layer of MAR-SAP sample act as a barrier to the elongation reducing the ductility when compared to un-nitrided samples.Once the tensile stress during the creep test overcomes the compressive residual stress, the cracks in the surface start to propagate, and the plastic deformation will concentrate in this region.The higher RMS roughness showed in Figure 4 by MAR-SAP sample indicates the higher roughness of the nitrided layer and may contribute to the presence of stress concentrators on the layer's surface to initiate the crack.Although this characteristic is negative to the creep resistance, the positive effect of oxidation barrier from the nitrided layer compensates the ductility loss, and the overall creep resistance is improved at the end.

Conclusions
The following concluding remarks could be drawn based on plasma nitrided and un-nitrided 300 grade maraging steel: • An uniform 50 μm thick surface layer of ε-Fe 3 N and γ'-Fe 4 N was formed, increasing the surface microhardness from 604 ± 18 HV to 1,140 ± 15 HV and RMS roughness in the nanometric scale from 52 nm to 71 nm, respectively for un-nitreded an nitrided regions.The continuous hardness decrease from surface to the core of the sample suggests the presence of two regions: one where the nitride phases are predominant and other with solution hardening by nitrogen atoms.The un-nitrided core remained un-altered, showing the typical hardness value and plate-like martensite microstructure; • Typical creep curves consisting of well-defined primary, secondary and tertiary stages were found.
Creep resistance of nitrided samples improved from 52-65% when compared to un-nitrided samples, and it seems to be associated with the continuous layer of iron nitrides that behave as a diffusion barrier for inward oxygen diffusion into the alloy reducing the oxidation rate.

Figure 1
Figure1is the cross-sectional image of MAR-SAP showing an uniform and continuous hardened nitrided layer of 50 μm thick and the typical martensite plate-like microstructure of maraging steels in the un-nitrided core.The X-ray diffraction (XRD) pattern in the 2θ ranging from 35-90º of MAR-SA and MAR-SAP samples are shown in Figure2.MAR-SA exhibit diffraction peaks only due to the martensitic phase α'-Fe, Figure2a.After plasma nitriding

Figure 1 :
Figure 1: Optical micrograph of the MAR-SAP showing the nitrided layer and typical plate-like martensite microstructure of maraging steels in the un-nitrided core.

Figure 6 :
Figure 6: Dependence of steady-state rate on applied stress at 650 ºC for MAR-SA 18 and MAR-SAP (the slope is n).

Figure 7 :
Figure 7: Dependence of steady-state rate on temperature at 500 MPa for MAR-SA 18 and MAR-SAP (the slope is -Q c /R).

Figure 9 :
Figure 9: Secondary electron SEM images showing the fracture morphology of the MAR-SA crept at 650 ºC and 300 MPa.(a) The dimpled rupture surrounded by shear lips (characteristic of ductile fracture).(b) Equiaxed dimples at the location enclosed in Fig. 9a.

Figure 10 :
Figure 10: Secondary electron SEM images showing the fracture surface of the MAR-SAP crept at 650 ºC and 300 MPa.(a) General morphology showing a cup and cone type of failure, the enclosed areas are the location where higher magnification in Figs.9b and 9c were taken.(b) Dimpled rupture area observed in the enclosed area marked as "b" in Fig. 9a.(c) A brittle type of fracture in the surface vicinity (nitrided layer), the enclosed area is where a higher magnification image in Fig. 9d was taken.(d) Cleavage surface and formation of microcracks close to the surface.

Figure 11 :
Figure 11: Secondary electron SEM image showing the circumferential cracks on the surface of plasma nitrided alloy crept at 650 ºC and 300 MPa with cracks perpendicular and cross-linked at a 45º angle relative to the tensile loading direction.

Figure 12 :
Figure 12: Correlation between steady-state creep rate (ε̇s) and time to fracture (t r ) for MAR-SA 18 and MAR-SAP from 550 to 650 ºC. 3

•
Based on the stress exponent (n), activation energy for creep (Q c ) and heterogeneous dislocation structure, it is concluded that the dominant creep mechanism is primarily controlled by dislocation climb.• Dominant type of failure was ductile showing a typical cup-and-cone fracture morphology, consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45º shear lip.The ductility of the nitrided samples are 17-40% lower, confirmed by Monkman-Grant relationship and fractographic analysis, and it is associated to the hard surface layer.