Effect of heating mode and electrochemical response on austenitic and ferritic stainless steels

Abstract

Powder metallurgical (P/M) processing has the main advantage of making near net shape products. Nowadays, in automobile industries, stainless steels have become the most promising material owing to their good corrosion resistance. In the current study, 316L and 434L stainless steel powders were sintered using microwave and conventional methods through powder metallurgy route. The effects of sintering modes on the microstructure, mechanical properties and corrosion responses of 316L and 434L stainless steel composites are investigated in detail. The results showed that the sample prepared through microwave sintering route exhibited significantly superior densification, higher hardness and better corrosion resistance as compared to the conventionally processed counterpart. On the whole, 316L composites showed better corrosion resistance than 434L stainless steels.     

Comprehensive Investigations of the Supersolidus Liquid-Phase Sintering of Two Plastic Mold Steels

The processing of plastics, particularly reinforced composites, necessitates the use of corrosion- and wear-resistant materials for tools that come into contact with the polymer. For such applications, plastic mold steels were developed that offer not only a good wear resistance due to the presence of carbides in a martensitic matrix, but also good corrosion resistance provided primarily by a sufficient amount of dissolved chromium. The common processing route for these high-alloyed materials is the hot isostatic pressing (HIP) of gas-atomized powders (PM-HIP). In this context, sintering plays an insignificant role, except for the processing of metal-matrix composites (MMCs). The development of novel wear- and corrosion-resistant MMCs based on plastic mold steels requires knowledge of the sintering behavior of prealloyed powders of such tool steels. It is well known that alloyed powders can be processed by supersolidus liquid-phase sintering (SLPS), a method leading to almost full densification and to microstructures without significant coarsening effects. In this work, two different gas-atomized powders of plastic mold steels were investigated by computational thermodynamics, thermal analysis, sintering experiments, and microstructural characterization. The results show that both powders can be sintered to almost full density (1 to 3 pct porosity) by SLPS in a vacuum or a nitrogen atmosphere. Experimental findings on the densification behavior, nitrogen uptake, and carbide volume fractions are in good agreement with calculations performed by computational thermodynamics.     

Effect of sintering temperature, heating mode and graphite addition on the corrosion response of austenitic and ferritic stainless steels

In recent years, sintered stainless steel with carbon addition has been proposed for potential use as exhaust flanges in automotive applications. The net-shaping requirements for such applications necessitate the use of powder metallurgical (P/M) processing route. However, due to the presence of porosity, most sintered steels have poor corrosion resistance. The present study compares the effect of sintering temperature, heating mode and graphite addition (up to 1.5%) on the densification and electrochemical response of both ferritic (434L) and austenitic (316L) stainless steels. The compacts were sintered in both conventional (radiatively heated) as well as microwave furnace. As compared to conventional sintering, samples consolidated in microwaves have higher densification (particularly at 1200°C) and exhibit better corrosion properties.     

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     

Studies on Direct Laser Cladding of SiC Dispersed AISI 316L Stainless Steel

In the present study, attempts have been made to develop SiC dispersed (5 and 20 wt pct) AISI 316L stainless steel matrix composite by direct laser cladding with a high power diode laser. Direct laser cladding has been carried out by melting the powder blends of AISI 316L stainless steel and SiC (5 and 20 wt pct) and, subsequently, depositing it on mild steel (0.15 pct C steel) in a layer by layer fashion to develop a coupon of 100 mm² × 10 mm dimension. A continuous, defect-free (microcracks and micro- or macroporosities), and homogeneous microstructure is formed, which consists of a dispersion of partially dissolved SiC (leading to formation of very low fraction of Cr₃C₂ and Fe₂Si) in grain-refined austenite. The microhardness of the clad layer increases from 155 VHN to 250 to 340 VHN (for 5 wt pct SiC dispersed) and 450 to 825 VHN (for 20 wt pct SiC dispersed) as compared to 155 VHN of commercially available AISI 316L stainless steel. The corrosion rate in 3.56 wt pct NaCl solution is significantly reduced in 5 wt pct SiC dispersed steel; however, 20 wt pct SiC dispersed steel showed a similar behavior as the commercially available AISI 316L stainless steel. The processing zone for the development of a defect-free microstructure with improved properties has been established.     

Effect of conventional and microwave sintering on the properties of yttria alumina garnet-dispersed austenitic stainless steel

The current study examines the effect of heating mode, temperature, and varying yttria alumina garnet (YAG) addition (5 and 10 wt pct) on the densification and properties of austenitic (316L) stainless steel. The straight 316L stainless steel and 316L-YAG composites were heated in a radiatively heated (conventional) and 2.45 GHz microwave sintering furnace. The compacts were consolidated through solid state as well as supersolidus sintering at 1200 °C and 1400 °C, respectively. Both 316L and 316L-YAG compacts couple with microwaves and heat to the sintering temperature rapidly (∼45 °C/min). The overall processing time was reduced by about 90 pct through microwave sintering. As compared to conventional sintering, compacts sintered in microwaves exhibit higher densification and finer microstructure but no corresponding improvement in mechanical properties and wear resistance. This has been correlated to elongated, irregular pore structure in microwave-sintered compacts.     

Injection moulding of 316L stainless steels reinforced with nanosize alumina particles

Abstract

This study aims to compare the effect of Al₂O₃ nanoparticle additions on the densification and mechanical properties of the injection moulded 316L stainless steels. The 316L stainless steel and Al₂O₃ nanoparticles were dry mixed and moulded using a wax based binder. The critical powder loading for injection moulding were 60 vol.-% for all samples. Debinding process was performed in solvent using thermal method. After the debinding process, the samples were sintered at 1405°C for 60 and 120 min under vacuum. Metallographic examination was conducted to determine the extend of densification and the corresponding microstructural changes. The sintered samples were characterised by measuring tensile strength, hardness and wear behaviour. Wear loss was determined for all the samples after wear testing. All the powders, fracture surfaces of moulded and sintered samples were examined using scanning electron microscope. The sintered density of straight as well as Al₂O₃ nanoparticles reinforced injection moulded 316L stainless steels increases with the increase in sintering time. The additions of Al₂O₃ nanoparticles improve the hardness and wear resistance with the increase of sintering time.     

Development of Duplex Stainless Steels by Field-Assisted Hot Pressing: Influence of the Particle Size and Morphology of the Powders on the Final Mechanical Properties

The feasibility of processing duplex stainless steels with promising properties using a powder metallurgical route, including the consolidation by field-assisted hot pressing, is assessed in this investigation. The influence of the particle size and morphology of the raw austenitic and ferritic powders on the final microstructure and properties is also evaluated for an austenitic content of 60 wt pct. In addition, the properties of a new microconstituent generated between the initial constituents are analyzed. The maximum sintered density (98.9 pct) and the best mechanical behavior, in terms of elastic modulus, nanohardness, yield strength, ultimate tensile strength, and ductility, are reached by the duplex stainless steel processed with austenitic and ferritic gas atomized stainless steel powders.

     

Effect of N addition on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels

The effect of N addition on the microstructure, tensile, and corrosion behaviors of CD4MCU (Fe-25Cr-5Ni-2.8Cu-2Mo) cast duplex stainless steel was examined in the present study. The slow strain rate tests were also conducted at a nominal strain rate of 1 × 10⁻⁶/s in air and 3.5 pct NaCl+5 pct H₂SO₄ solution for studying the stress corrosion cracking (SCC) behavior. It was observed that the volume fraction of austenitic phase in CD4MCU alloy varied from 38 to 59 pct with increasing nitrogen content from 0 to 0.27 wt. pct. The tensile behavior of CD4MCU cast duplex stainless steels, which tended to vary significantly with different N contents, appeared to be strongly related to the volume changes in ferritic and austenitic phases, rather than the intrinsic N effect. The improvement in the resistance to general corrosion in 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution was notable with 0.13 pct N addition. The further improvement was not significant with further N addition. The resistance to SCC of CD4MCU cast duplex stainless steels in 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution, however, increased continuously with increasing N content. The enhancement in the SCC resistance was believed to be related to the volume fraction of globular austenitic colonies, which tended to act as barriers for the development of initial pitting cracks in the ferritic phase into the sharp ones.     

Electrochemical behaviour of sintered,yttria reinforced ferritic stainless steels

Abstract

The electrochemical behaviour of 434L ferritic stainless steels produced by the powder metallurgy (PM) route has been studied. The effects of sintering temperature (1250 and 1400°C) and dispersoid addition (10%Y₂O₃) have been investigated by potent iodynamic polarisation and electrochemical impedance studies in 0·05 M H₂SO₄ solution. All the alloys exhibited active-passive behaviour in the electrolyte. The critical current density for passivation was lower in the case of the PM samples when compared with 430, which has been attributed to the presence of Mo in the samples. The passive current density of the PM samples correlated with the sintered porosities. All the other passivation parameters were similar for the materials. The nature of passive film was investigated using electrochemical impedance spectroscopy. A lower capacitance (i.e. more corrosion resistant) surface film was obtained after sintering at the higher temperature. Yttria dispersoids in ferritic stainless steel decreased the corrosion resistance of the surface film after sintering at the higher temperature, whereas they did not significantly affect the surface film behaviour after lower sintering temperature. The results have been correlated to microstructural parameters of the PM materials.     

Structure and properties of corrosion and wear resistant Cr-Mn-N steels

Steels containing about 12 pct Cr, 10 pct Mn, and 0.2 pct N have been shown to have an unstable austenitic microstructure and have good ductility, extreme work hardening, high fracture strength, excellent toughness, good wear resistance, and moderate corrosion resistance. A series of alloys containing 9.5 to 12.8 pct Cr, 5.0 to 10.4 pct Mn, 0.16 to 0.32 pct N, 0.05 pct C, and residual elements typical of stainless steels was investigated by microstructural examination and mechanical, abrasion, and corrosion testing. Microstructures ranged from martensite to unstable austenite. The unstable austenitic steels transformed to α martensite on deformation and displayed very high work hardening, exceeding that of Hadfield’s manganese steels. Fracture strengths similar to high carbon martensitic stainless steels were obtained while ductility and toughness values were high, similar to austenitic stainless steels. Resistance to abrasive wear exceeded that of commercial abrasion resistant steels and other stainless steels. Corrosion resistance was similar to that of other 12 pct Cr steels. Properties were not much affected by minor compositional variations or rolled-in nitrogen porosity. In 12 pct Cr-10 pct Mn alloys, ingot porosity was avoided when nitrogen levels were below 0.19 pet, and austenitic microstructures were obtained when nitrogen levels exceeded 0.14 pct.     

Wear Behavior of the Lead-Free Tin Bronze Matrix Composite Reinforced by Carbon Nanotubes

Copper-coated carbon nanotubes were prepared by the electroless plating route. The structure and component of copper/carbon tubes were characterized using a transmission electron microscope and energy dispersive spectrometer. The results show that the surface of the carbon tubes was covered by the copper particles. Copper/carbon tubes were used as the substitute of part of tin and all of lead in the tin bronze matrix, and the tribological properties of carbon nanotube-reinforced Cu-4 wt pct Sn-6 wt pct Zn composites were studied. The effects of the carbon nanotube volume fraction and sliding distance in unlubricated ball-on-disc wear test were investigated. The 3 vol pct carbon nanotube-reinforced Cu-4 wt pct Sn-6 wt pct Zn composite shows the Vickers hardness of 126.9, which is approximately 1.6 times higher than that of Cu-6 wt pct Sn-6 wt pct Zn-3 wt pct Pb tin bronze. The wear rate and average friction coefficients of 3 vol pct carbon nanotube-reinforced Cu-4 wt pct Sn-6 wt pct Zn composite were lower than those of the Cu-6 wt pct Sn-6 wt pct Zn-3 wt pct Pb tin bronze, respectively.     

Evaluation of corrosion response of sintered ferritic stainless steels using potentiodynamic polarization and electrochemical impedance spectroscopy techniques

The present study investigates the effect of sintering temperature on the electrochemical response of ferritic (434L) stainless steels determined through potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS) studies. To understand the corrosion mechanism, the EIS was done at open circuit potential (OCP), passive region (500mV) and near breakdown potential region of polarization curve of 434L stainless steel. The compacts sintered at higher temperature (1400°C) exhibit better corrosion resistance. To explain the EIS results, an electrical circuit has been conceptualized at open circuit potential (OCP) that indicates the presence of two time constants. This can be attributed to the presence of Electrical Double Layer (EDL) and formation of a protective passive during corrosion. EIS results on compacts done at 500mV shows only one time constant which suggests nearly complete passivation of the surface due to the formation of passive film.     

Effect of different Cr contents on tensile and corrosion behaviors of 0.13 pct N-containing CD4MCU cast duplex stainless steels

The tensile and corrosion behaviors of 0.13 pct N-containing CD4MCU cast duplex stainless steels with different Cr contents ranging from 23 to 28 pct were examined in the present study. The polarization tests were conducted in 3.5 pct NaCl + 5 pct H₂SO₄ aqueous solution for general corrosion resistance, and the in-situ slow strain rate (SSR) tests were also conducted in air and 3.5 pct NaCl + 5 pct H₂SO₄ aqueous solution to quantify the resistance to stress corrosion cracking (SCC) of the three materials. A substantial microstructural change in 0.13 pct N-containing CD4MCU cast duplex stainless steel was observed with different Cr contents, which in turn affected the tensile and corrosion behaviors significantly. Tensile behavior of 0.13 pct N-containing CD4MCU cast duplex stainless steel, for example, varied in a nonlinear manner with different Cr contents due to the volume change of hard ferritic phase and the presence of the second precipitates of soft austenitic phase in the ferrite matrix. The beneficial effect of Cr for improving the general corrosion and the SCC resistances was largely overshadowed by this variation in microstructural characteristics. The relationship between the microstructural evolution and the tensile and corrosion behavior of 0.13 pct N-containing CD4MCU cast duplex stainless steels with different Cr contents was discussed based on the optical microscopy and scanning electron microscopy (SEM) micrographic and fractographic observations.     

Retardation of intermetallic phase formation in experimental superferritic stainless steels

While superferritic stainless steels containing 29 pct chromium possess excellent resistance to corrosion, they may, under certain conditions, be embrittled by the precipitation of intermetallic phases. The extent to which the precipitation reactions can be retarded by alloying additions of aluminum and copper has been evaluated. It was found that additions of aluminum to an Fe-29 pct Cr-4 pct Mo-1.5 pct Ni base alloy suppress the precipitation of the undesirable sigma and chi intermetallic phases, but additions of up to 3 pct aluminum promote 475 ‡C embrittlement. Additions of copper slightly reduce the precipitation of sigma and chi phases under most conditions but enhance 475 ‡C embrittlement. The resistance to corrosion in 10 pct H₂SO₄ and 10 pct FeCl₃ was assessed. All the aluminum-containing alloys performed significantly better in H₂SO₄ than the base alloy; however, large additions of aluminum had a deleterious effect on the pitting resistance in FeCl₃. Additions of copper improved the resistance to FeCl₃ and lowered the rate of corrosion in the H₂SO₄ solution used.     

Study of Heterogeneous Nucleation of Eutectic Si in High-Purity Al-Si Alloys with Sr Addition

Al-5 wt pct Si master-alloys with controlled Sr and/or P addition/s were produced using super purity Al 99.99 wt pct and Si 99.999 wt pct materials in an arc melter. The master-alloy was melt-spun resulting in the production of thin ribbons. The Al matrix of the ribbons contained entrained Al-Si eutectic droplets that were subsequently investigated. Differential scanning calorimetry, thermodynamic calculations, and transmission electron microscopy techniques were employed to examine the effect of the Sr and P additions on eutectic undercoolings and nucleation phenomenon. Results indicate that, unlike P, Sr does not promote nucleation. Increasing Sr additions depressed the eutectic nucleation temperature. This may be a result of the formation of a Sr phase that could consume or detrimentally affect potent AlP nucleation sites.     

Copper,Boron, and Cerium Additions in Type 347 Austenitic Steel to Improve Creep Rupture Strength

Type 347 austenitic stainless steel (18Cr-12Ni-Nb) was alloyed with copper (3 wt pct), boron (0.01 to 0.06 wt pct), and cerium (0.01 wt pct) with an aim to increase the creep rupture strength of the steel through the improved deformation and cavitation resistance. Short-term creep rupture strength was found to increase with the addition of copper in the 347 steel, but the long-term strength was inferior. Extensive creep cavitation deprived the steel of the beneficial effect of creep deformation resistance induced by nano-size copper particles. Boron and cerium additions in the copper-containing steel increased its creep rupture strength and ductility, which were more for higher boron content. Creep deformation, grain boundary sliding, and creep cavity nucleation and growth in the steel were found to be suppressed by microalloying the copper-containing steel with boron and cerium, and the suppression was more for higher boron content. An auger electron spectroscopic study revealed the segregation of boron instead of sulfur on the cavity surface of the boron- and cerium-microalloyed steel. Cerium acted as a scavenger for soluble sulfur in the steels through the precipitation of cerium sulfide (CeS). This inhibited the segregation of sulfur and facilitated the segregation of boron on cavity surface. Boron segregation on the nucleated cavity surface reduced its growth rate. Microalloying the copper-containing 347 steel with boron and cerium thus enabled to use the full extent of creep deformation resistance rendered by copper nano-size particle by increase in creep rupture strength and ductility.     

Grain Refining of 409L Ferritic Stainless Steel Using Fe-Ti-N Master Alloy

A deliberately prepared Fe-Ti-N master alloy was employed as a grain refiner for the 409L ferritic stainless steel in an attempt to refine the as-cast solidification structure. It was found that the average equiaxed grain size of the sample deceased from 1503 to 303 μm and the proportion of equiaxed grain zone increased from 14 to 100 pct with increasing the Fe-Ti-N master alloy addition level to 2.5 wt pct. Hence, this method may have good prospects for industrial applications.