Microstructural Characterization and Properties Evaluation of Ni-Based Hardfaced Coating on AISI 304 Stainless Steel by High Velocity Oxyfuel Coating Technique

The present study concerns a detailed investigation of microstructural evolution of nickel based hardfaced coating on AISI 304 stainless steel by high velocity oxy-fuel (HVOF) deposition technique. The work has also been extended to study the effect of coating on microhardness, wear resistance and corrosion resistance of the surface. Deposition has been conducted on sand blasted AISI 304 stainless steel by HVOF spraying technique using nickel (Ni)-based alloy [Ni: 68.4 wt pct, chromium (Cr): 17 wt pct, boron (B): 3.9 wt pct, silicon (Si): 4.9 wt pct and iron (Fe): 5.8 wt pct] of particle size 45 to 60 ??m as precursor powder. Under the optimum process parameters, deposition leads to development of nano-borides (of chromium, Cr₂B and nickel, Ni₃B) dispersion in metastable and partly amorphous gamma nickel (??-Ni) matrix. The microhardness of the coating was significantly enhanced to 935 VHN as compared to 215 VHN of as-received substrate due to dispersion of nano-borides in grain refined and partly amorphous nickel matrix. Wear resistance property under fretting wear condition against WC indenter was improved in as-deposited layer (wear rate of 4.65 × 10^?7 mm³/mm) as compared to as-received substrate (wear rate of 20.81 × 10^?7 mm³/mm). The corrosion resistance property in a 3.56 wt pct NaCl solution was also improved.     

An approach to developing an alternative hot work die steel

This paper evaluates an approach to developing a steel which combines resistance to softening on tempering with an economical use of alloy elements. To obtain the desired tempering behavior advantage has been taken of the ability of non-carbide forming elements to enhance the secondary hardening response and the tendency of vanadium additions to reduce the coarsening rate of Mo₂C. Five alloys were investigated; these were a base secondary hardening steel, that base steel modified by the addition of 2 wt pct silicon and by the combined addition of 1 wt pct silicon and 1 wt pct aluminum. To these two modified alloys were made additions of 0.4 wt pct vanadium. It was found that both types of additions without vanadium enhance the secondary hardening response to the same significant degree. Both of these alloys soften rapidly as the tempering temperature is increased above 600 °C. However, with the addition of vanadium, hardnesses over R_gC 50 are obtained after tempering at 650 °C. While silicon additions appear indispensable to this resistance to softening on tempering, silicon also favors the retention of primary carbides after austenitizing and, if present in sufficient amounts, can cause brittle intergranular fracture after tempering at high temperatures.     

The effect of nitrogen on stacking fault energy of Fe-Ni-Cr-Mn steels

The effect of nitrogen content on stacking fault energy (SFE) has been measured in a series of Fe-21Cr-6Ni-9Mn alloys. Stacking fault energies were determined from node measurements using weak beam imaging techniques in transmission electron microscopy. Nitrogen additions lower the SFE from 53 mJ/m² at 0.21 wt pct to 33 mJ/m² at 0.24 wt pct. Further increases to 0.52 wt pct do not markedly change the SFE. Carbon and silicon had no effect on SFE in the ranges 0.010 to 0.060 wt pct C and 0.17 to 0.25 wt pct Si. The shift in SFE from 0.21 to 0.24 wt pct N is accompanied by a transition to a more planar plastic deformation mode. The sharp transition precludes the use of linear regression analysis for relating SFE to nitrogen content in this class of alloys.     

Magnetic properties of maraging steels in relation to nickel concentration

Magnetic properties of maraging steels have been investigated as a function of nickel concentration. The alloys nickel content varied from 12 to 24 wt pct, while other alloying constituents were kept at a level maintained in the 18Ni-2400 MPA-grade maraging steel. The magnetic properties were determined following aging for 1 hour in the temperature range of 450 °C to 750 °C. In every alloy investigated, the coercive field increased with aging temper-ature, reaching a maximum around 670 °C ± 30 °C. The saturation magnetization values were lowest around temperatures where maximum coercive field was observed. The coercive field increased from ∼55 to ∼ 175 Oe (∼4380 to ∼ 13,900 amp/meter) and the corresponding sat-uration magnetization decreased from ∼18,500 to ∼ 4000 G (∼1.85 to ∼0.4 T) in the alloys containing 12 and 24 wt pct Ni, respectively. The reverted austenite increased from 25 vol pct at 12 wt pct Ni to 100 vol pct at 24 wt pct Ni. The hardness and Charpy impact strength of the alloys have also been determined. An attempt has been made to correlate magnetic properties with different phase transformations occurring in maraging steels.     

Strengthening of steel by the method of spraying oxide particles into molten steel stream

A new technique, Spray-Dispersion Method, which produces a steel containing homogeneously dispersed fine oxide particles sprayed from outside into the molten steel stream has been developed. The conditions for the distribution of particles in solid steel, and the mechanical properties of A1₂O₃- or ZrO₂-dispersed steels were studied. The homogeneous distribution of fine oxide particles is obtained by the addition of a certain suitable controlling element which lowers the contact angle of molten steel on various oxides and the interfacial tension at the oxide—molten steel interface. Among others columbium (niobium) was found to be the most effective on decreasing the average particle size of sprayed oxides. Because of fine dispersion of particles of less than 120 nm, the strength of steels increased with their volume fractions; for example, 1.15 vol pct ZrO₂ causes an increment of 79 MN/m² in proof strength and 94 MN/m² in tensile strength. For practical applications, the Spray Dispersion Method makes it possible to produce 18 chromium-8 nickel austenitic stainless steels dispersion-strengthened by A1₂O₃ or ZrO₂ particles, and ThO₂-dispersed nickel with an average particle size of 61.4 nm and a volume fraction of 5.1 pct.     

Phase transformational kinetics and hardenability of alloyed medium-carbon steels

The phase transformational kinetics and hardenability of 0.4 pct C steels were studied as influenced by alloying elements, singly and in combination. Sixteen series of steels, each containing up to about 0.75 pct Mo, were prepared by laboratory induction air melting. The base steels contained manganese, chromium, nickel, silicon, and combinations of these elements as alloy additions. Continuous cooling transformation diagrams, hardenability diagrams, and diagrams of the effects of alloying on the beginning of bainitic transformation and the beginning of ferritic transformation were established. The essential findings of the research program are summarized in this paper to establish a better understanding of alloy interactions and their effects on structure and properties. The effects of combinations of alloying elements in delaying the bainitic and ferritic-pearlitic transformations, and in increasing the hardenability cannot be predicted from the effects of each element alone. Furthermore, in most of the multi-alloy systems studied, molybdenum has been found to enhance the effectiveness of the other alloying elements present. Continuous cooling transformation diagrams clarify these relationships and together with the hardenability data provide a basis for predicting strength levels that can be obtained for steel products of varying section size. The 1.5 pct Si-1.5 pct Mn-1.4 pct Ni-0.7 pct Cr-Mo steels exhibit the maximum suppression of bainitic transformation and are closely followed by similar steels without nickel. The wide time range of the martensitic transformation in these steels qualifies them for consideration for air-hardenable constructional components of substantial section size. This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee.     

Thermodynamic prediction of the ae3 temperature of steels with additions of Mn,Si, Ni,Cr, Mo,Cu

Published binary phase diagrams and activity data for iron and binary and ternary alloys have been used to evaluate the general linear series expansion of the activity coefficient and the standard free energy changes and these have been employed in turn for the accurate thermodynamic determination of the Ae₃ temperature in steels with additions of Mn, Si, Ni, Cr, Mo, and Cu. A computer program accurate for total additions up to 7.0 wt pct and an analytic formula accurate for total additions up to 2.5 wt pct have been developed. The predicted Ae₃ temperatures compare favorably with observations on over 200 steels from international compendia. It is demonstrated that existing linear regression formulae are incorrect and that for low alloy additions they should be replaced by linear expressions with carbon concentration dependent coefficients.     

Strain rate sensitivity and ductile-brittle behavior of polycrystalline Fe-Si alloys with 2.5, 3.5, and 4.5 wt pct Si

The strain rate sensitivity of the tensile properties of polycrystalline Fe-Si alloys with 2.5, 3.5, or 4.5 wt pct Si and a C+N content of 0.005 to 0.010 wt pct has been determined at room temperature. From these and previous results the influence of silicon on the athermal and thermal components of the yield (proof) stress of ferrite has been deduced. Both components increase with silicon content, but at the highest strain rates (and lowest temperatures) the thermal component varies little with silicon content and is smaller than that reported for pure iron. Transmission electron microscopy of ductile alloys has shown that the density and arrangements of dislocations are not greatly influenced by silicon content or strain-rate within the range investigated. At room temperature the dislocation velocity exponent,m*, increases with silicon content, being 4.5, 7.4, and 10.6 (±1.0) in the three alloys. For the grain size studied (5 to 7 ASTM) it appears that the stress for yielding by glide must not exceed about 700 MN/m² (100 ksi) if brittle behavior is to be avoided. The combinations of temperature and strain rate at which the ductile-brittle transition occurs are indicated for different silicon contents and different grain sizes. Formerly with the same Institute     

Carbides in high-speed steels containing silicon

The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M₂C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undis-solved M₆C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M₆C carbides. The amount of MC and M₂C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.     

Interrelations of cooling rate,microstructure, and mechanical properties in four HSLA steels

The present study was carried out on four steels containing 0.1 pct C-1.5 pct Mn-0.003 pct B* in common, with additions of 1 pct Cr, 0.5 pct Mo, 0.25 pct Mo + 1 pct Cr, 0.2 pct Ti + 1 pct Cr. They were designated, accordingly, as Cr, Mo, Mo-Cr, and Cr-Ti steels. All the steels exhibited a complete lath martensite microstructure with thin interlaths of retained austenite (≈0.05 pct) in the quenched condition. The normalized microstructures, granular bainite, contained massive areas of ferrite and granules of bainite laths. Both microconstituents contained a fine dispersion of cementite particles (size ≈50 Å) together with high dislocation densities. A mechanism explaining their for-mation has been given. The Cr steel, due to its low hardenability, showed in addition polygonal ferrite in the neighborhood of the so-called M-A constituent (twinned martensite and/or austenite). The annealed microstructure (using a cooling rate of 0.033 °C s^?1) of the Cr steel consisted of coarse ferrite-pearlite. Addition of 0.2 pct Ti to the Cr steel markedly refined the structure, whereas an addition of 0.25 pct Mo altered the microstructure to ferrite-lower bainite. In the 0.5 pct Mo steel, polygonal ferrite was found to be completely missing. The mechanical properties of the four steels after quenching, normalizing, and annealing were investigatedvia hardness and tensile test mea-surements. An empirical equation, relating the ultimate tensile strength to the steel composition, for steels that had granular bainite microstructures in the normalized condition, was proposed. The fracture surfaces exhibited cleavage and variable-size dimples depending on the microstructure and steel composition.     

Phase transformational kinetics and hardenability of low-carbon,boron-treated steels

The phase transformations and hardenability of 0.1 pct C boron-treated and boron-free steels containing Mn, Cr, Ni, or Cr plus Ni, and up to 1 pct Mo were studied. Continuous cooling transformation diagrams, hardenability characteristics, and diagrams of the ferrite start half-cooling time vs alloying were established. An unalloyed 0.1 pct C steel transforms diffusionally in the ferritic-pearlitic range when cooled from an austenitizing temperature, with a negligible contribution of the intermediate (bainitic) transformation occurring at very high rates of cooling. Molybdenum extends the range of the bainitic transformation and markedly delays the decomposition of austenite in the ferritic-pearlitic range. Boron treatment of the unalloyed (molybdenum-free) 0.1 pct C steel permits bainite formation over a wider range of fast cooling programs. At lower rates of cooling, the steel transforms diffusionally into ferrite and pearlite . Alloying additions of Mn, Cr, or Ni result in a slightly higher proportion of the bainitic transformation, which may occur over a wider range of cooling programs. When both nickel and chromium are present, a modest synergistic effect on the delay of the ferritic-pearlitic transformation may be noted. Introduction of molybdenum into all of the boron-treated 0.1 pct C steels strongly delays the decomposition of austenite into ferrite-pearlite structures and vastly expands the range of cooling programs that result in the formation of bainitic structures. In this important action, molybdenum is assisted to a smaller degree by alloying additions of manganese and chromium, and to a greater degree by nickel and chromium plus nickel. In all the steels studied, the alloying elements lower the temperatures of the bainitic transformation, thereby explaining, at least partly, the somewhat higher hardness for any specified cooling program. The observed beneficial effects of boron, molybdenum, and other alloying elements on the phase transformational behavior on continuous cooling are reflected in terms of higher hardenability.     

High temperature embrittlement of NI and Ni-Cr alloys by trace elements

The effects of Sb, Sn, and Zr additions on the creep properties of Ni and Ni + 20 pct Cr are reported. Antimony and tin additions (~1 wt pct) induce extensive grain boundary cavitation in nickel, while smaller antimony additions had little effect on Ni + 20 pct Cr. Addition of 0.11 pct Zr to Ni + 20 pct Cr greatly inhibited grain boundary cavitation and reduced its Coble creep rate. Auger electron spectroscopy of cavitated specimens provided direct evidence of impurity segregation to cavity surfaces. Residual sulfur segregated most strongly, and was observed on cavity surfaces in all cavitated specimens. Tin segregated somewhat less intensely than sulfur, and antimony segregated only slightly. Segregation of antimony and sulfur to uncavitated portions of Ni + 1 pct Sb grain boundaries was also observed. These results are discussed in terms of segregation effects on energetic and transport properties of the grain boundaries and cavity surfaces. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

Replacement of Ni by Mn in High-Ni-Containing Austenitic Cast Steels used for Turbo-Charger Application

High-temperature tensile properties of austenitic cast steels fabricated by replacing Ni by Mn in a 20 wt pct Ni-containing steel were investigated. In a steel where 8 wt pct Ni was replaced by 9.2 wt pct of Mn, 17.4 and 9.8 pct of ferrite existed in equilibrium phase diagrams and actual microstructures, respectively, because a role of Mn as an austenite stabilizer decreased, and led to deterioration of high-temperature properties. When 2 to 6 wt pct Ni was replaced by 2.3 to 6.9 wt pct Mn, high-temperature properties were comparable to those of the 20 wt pct Ni-containing steel because ferrites were absent, which indicated the successful replacement of 6 wt pct Ni by Mn, with cost reduction of 27 pct.     

Chemistry and Properties of Medium-Mn Two-Stage TRIP Steels

Eight medium manganese steels ranging from 10 to 15 wt pct Mn have been produced with varying levels of aluminum, silicon, and carbon to create steels with varying TRIP (transformation-induced plasticity) character. Alloy chemistries were formulated to produce a range of intrinsic stacking fault energies (ISFE) from − 2.2 to 13.3 mJ/m² when calculated at room temperature for an austenitic microstructure having the nominal alloy composition. Two-stage TRIP behavior was documented when the ISFE of the γ-austenite phase was 10.5 mJ/m² or less, whereas an ISFE of 11.9 mJ/m² or greater exhibited TWIP (twin-induced plasticity) with single-stage TRIP to form α-martensite. Properties were measured in both hot band (hot rolled) and batch annealed (hot rolled, cold rolled, and annealed) conditions. Hot band properties were influenced by the Si/Al ratio and this dependence was related to incomplete recovery during hot working for alloys with Si/Al ratios greater than one. Batch annealing was conducted at 873 K (600 °C) for 20 hours to produce ultrafine-grained microstructures with mean free slip distances less than 1 μm. Batch-annealed materials were found to exhibit a Hall–Petch dependence of the yield strength upon the mean free slip distance measured in the polyphase microstructure. Ultimate tensile strengths ranged from 1450 to 1060 MPa with total elongations of 27 to 43 pct. Tensile ductility was shown to be proportional to the sum of the products of volume fraction transformed times the volume change associated for each martensitic transformation. An empirical relationship based upon the nominal chemistry was derived for the ultimate tensile strength and elongation to failure for these batch-annealed steels. Two additional alloys were produced based upon the developed understanding of these two-stage TRIP steels and tensile strengths of 1150 MPa with 58 pct total elongation and 1400 MPa and 32 pct ductility were achieved.

     

Stress corrosion cracking of stainless steels in NaCl solutions

The metallurgical influences on the stress corrosion resistance of many commercial stainless steels have been studied using the fracture mechanics approach. The straight-chromium ferritic stainless steels, two-phase ferritic-austenitic stainless steels and high-nickel solid solutions (like alloys 800 and 600) investigated are all fully resistant to stress corrosion cracking at stress intensity (K₁) levels ≤ MN • m-^3/2 in 22 pct NaCl solutions at 105 °C. Martensitic stainless steels, austenitic stainless steels and precipitation hardened superalloys, all with about 18 pct chromium, may be highly susceptible to stress corrosion cracking, depending on heat treatment and other alloying elements. Molybdenum additions improve the stress corrosion cracking resistance of austenitic stainless steels significantly. The fracture mechanics approach to stress corrosion testing of stainless steels yields results which are consistent with both the service experience and the results from testing with smooth specimens. In particular, the well known “Copson curve” is reproduced by plotting the stress corrosion threshold stress intensity (AT_ISCC) vs the nickel content of stainless steels with about 18 pct chromium. Formerly with the BBC Brown Boveri Company, Baden, Switzerland     

Structure,strength, and fracture of electrodeposited nickel and Ni−Co alloys

Massive electrodeposits of nickel and Ni?Co alloys ranging up to 43 pct Co were examined microstructurally and tested to determine tensile properties and static and dynamic fracture toughness. Specimens were also tested after being annealed at 575 K. Annealing increased grain size, decreased yield, and ultimate strengths, and increased ductility and dynamic toughness. The as-plated Ni-43 Co was the only material to exhibit validK _IC values, averaging about 38 MN/m^3/2. In instrumented dynamic tests on precracked Charpy bars, the same material exhibited aK _Id of 50 MN/m^3/2. The yield strength of the Ni-43 Co alloy was 1154 MN/m². All the materials tested showed dimpled, ductile rupture fracture surfaces. The Hall-Petch behavior of the nickel indicated that it is much easier to initiate flow in normal grain boundary structures than in structures composed of dislocation cell walls.     

FeS-MnS phase relationships in the presence of excess iron

The FeS-MnS system is reexamined, both with and without excess iron. When excess iron is present, as is true for sulfide inclusions within steel, the pseudobinary reveals a peritectic rather than the previously assumed eutectic invariant. The maximum solubility limits (997 ± 3°C, or 1270 K) in the two solid phases are: a) 7.5 wt pct MnS in FeS, and b) 73.5 wt pct FeS in MnS. The peritectic liquid contains 66 wt pct Fe, ∼34 wt pct S, and ∼0.4 wt pct Mn. The two solid sulfide phases are nearly stoichiometric in the presence of excess iron; the Fe-richer sulfide is metal-deficient in the absence of a metallic iron phase. Based on this study, it is possible to be more specific than heretofore about the Fe-FeS-MnS-Mn region of the Fe-Mn-S ternary. In addition to the presence of a peritectic, it was concluded that the miscibility gap does not cross the univariant line between primary metal and (Mn,Fe)S phases. The peritectic liquid and the Mn-richer solid sulfide equilibrate with a metal containing ≤ 0.36 wt pct Mn. These data help explain the Mn/s ratios required to avoid hot-shortness in regular and resulfurized plain-carbon steels. G. S. MANN, formerly Graduate Student This is a part of the dissertation submitted by G. S. Mann for his Ph.D. at the University of Michigan     

Precipitation and fine structure in medium-carbon vanadium and vanadium/niobium microalloyed steels

Hot-rolled and continuously cooled, medium-carbon microalloyed steels containing 0.2 or 0.4 pct C with vanadium (0.15 pct) or vanadium (0.15 pct) plus niobium (0.04 pct) additions were investigated with light and transmission electron microscopy. Energy dispersive spectroscopy in a scanning transmission electron microscope was conducted on precipitates of the 0.4 pct C steel with vanadium and niobium additions. The vanadium steels contained fine interphase precipitates within ferrite, pearlite nodules devoid of interphase precipitates, and fine ferritic transformation twins. The vanadium plus niobium steels contained large Nb-rich precipitates, precipitates which formed in cellular arrays on deformed austenite substructure and contained about equal amounts of niobium and vanadium, and V-rich interphase precipitates. Transformation twins in the ferrite and interphase precipitates in the pearlitic ferrite were not observed in either of the steels containing both microalloying elements. Consistent with the effect of higher C concentrations on driving the microalloying precipitation reactions, substructure precipitation was much more frequently observed in the 0.4C-V-Nb steel than in the 0.2C-V-Nb steel, both in the ferritic and pearlitic regions of the microstructure. Also, superposition of interphase and substructure precipitation was more frequently observed in the high-C-V-Nb steel than in the similar low-C steel.     

A melting and solidification study of alloy 625

The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M₆C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M₆C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m _L , m _S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.