Commercial Stainless Steel Research Articles

Pit Initiation Behavior at Sulfide Particles in Sintered Stainless Steels

The pit initiation sites for commercial stainless steels have been attributed to sulfide inclusions such as MnS. The chemical composition of the inclusions affects the pitting corrosion resistance of stainless steels. Systematic investigation on the relationship between the chemical composition of the inclusions and the pitting corrosion resistance is required. However, it is difficult to prepare a series of the steel specimens of which the chemical composition of the inclusions is precisely controlled. In this study, spark plasma sintering was used to fabricate stainless steel specimens containing sulfide particles, and a microelectrochemical technique was applied to analyze the effect of the chemical composition of the sulfides on the pitting corrosion resistance of stainless steels. A type 304 stainless steel powder was mixed with a MnS powder to prepare the sintered specimen containing 0.03 mass% S. The particle sizes of the stainless steel and the MnS powder were –100 mesh and –325 mesh, respectively. The mixed powders were loaded into a cylindrical graphite die and were compressed using a hydraulic press. The pressed compact was spark plasma sintered in vacuum, under uniaxial pressure at 30 MPa. The sintered specimen was ground with SiC papers and then was polished with an 1 µm diamond paste. Potentiodynamic polarization was conducted in naturally aerated 0.1 M NaCl (pH 5.5) at 298 K. The specimen surface was masked to make an electrode area of ca. 100 µm × 100 µm containing a sulfide particle. All the potentials reported in this study refer to the Ag/AgCl (3.33 M KCl) electrode (0.206 V vs. standard hydrogen electrode at 298 K). The potential scan rate was 3.8 × 10-4 V/s. The anodic polarization curve was measured in 0.1 M NaCl to characterize the pit initiation behavior at the MnS particle in the sintered stainless steel. Polarization was started at –0.2 V and was stopped after the large increase in the current density was measured. The electrode area was observed using an optical microscope after polarization. The MnS particle partly dissolved, and the pit was found at the boundary between the particle and the steel matrix. This suggests that the large current increase was attributed to the pit initiation at the particle. It is expected that the sintered specimen can be used to investigate the pit initiation behavior at the sulfides in stainless steels.

Thermodynamics of an austenitic stainless steel (AISI-348) under in situ TEM heavy ion irradiation

The stability of the face-centred cubic austenite (γ-Fe) phase in a commercial stainless steel (AISI-348) was investigated through in situ transmission electron microscopy (TEM) with heavy ion irradiation at 1073 K up to a fluence of 1.3×1017 ions⋅cm−2 (corresponding to a dose of 46 dpa). The γ-Fe phase was observed to decompose at a fluence of around 7.8×1015 ions⋅cm−2 (3 dpa) when a new phase nucleated and grew upon increasing irradiation dose. Scanning transmission electron microscopy (STEM) with energy dispersive X-ray (EDX) spectroscopy and multivariate statistical analysis (MVSA) were used to characterise the irradiated specimens. The combination of such experimental techniques with calculated equilibrium phase diagrams using the CALPHAD method led to the conclusion that the new phase formed upon irradiation is the body-centred cubic Cr-rich α' phase. At the nanoscale, precipitation of M23C6 (τ-carbide) was also observed. The results indicate that ion irradiation can assist the austenitic stainless steel to reach a non-equilibrium state similar to a calculated equilibrium state observed at lower temperatures.

Feasibility of preparing additive manufactured porous stainless steel felts with mathematical micro pore structure as novel catalyst support for hydrogen production via methanol steam reforming

In this paper, an additive manufacturing prepared porous stainless steel felt (AM-PSSF) is proposed as a novel catalyst support for hydrogen production via methanol steam reforming (MSR). In the method, 316 L stainless steel powder with diameter of 15–63 μm is processed by the additive manufacturing technology of selective laser melting (SLM). To accomplish the preparation, the reforming chamber where the AM-PSSF is embedded is firstly divided into an all-hexahedron mesh. Then, the triply periodic minimal surface (TPMS) unit with mathematical form, high interconnectivity and large specific surface area is mapped into the hexahedrons based on shape function, forming the fully connected three-dimensional (3D) micro pore structure of the AM-PSSF. By correlating the mathematical parameter and the porosity of the TPMS unit, and taking into account the SLM process, the porosity of the AM-PSSF is well controlled. Based on the designed 3D pore structure model, the AM-PSSF is produced using standard SLM process. The application of the AM-PSSF as catalyst support for hydrogen production through MSR indicates that: 1) both the naked and catalyst-coated AM-PSSF have the characteristics of high porosity, large specific surface area and high connectivity; 2) the MSR hydrogen production performance of the AM-PSSF is better than that of the commercial stainless steel fiber sintered felt. The feasibility of AM-PSSF as catalyst support for MSR hydrogen production may pave a better way to balance different requirements for catalyst support, thanks to the excellent controllability provided by AM on both the external shape and the internal pore structure, and to the produced rough surface morphology that benefits the catalyst adhesion strength. In addition, catalyst support with pore structures that are more accommodated with the flow field and the reaction rate of MSR reaction may be prepared in future, since the entire catalyst support structure, from macro scale to micro scale, is under control.

Composition-Nanostructure Steered Performance Predictions in Steel Wires.

Neutron scattering in combination with scanning electron and atomic force microscopy were employed to quantitatively resolve elemental composition, nano- through meso- to metallurgical structures and surface characteristics of two commercial stainless steel orthodontic archwires—G&H and Azdent. The obtained bulk composition confirmed that both samples are made of metastable austenitic stainless steel type AISI 304. The neutron technique’s higher detection sensitivity to alloying elements facilitated the quantitative determination of the composition factor (CF), and the pitting resistance equivalent number (PREN) for predicting austenite stability and pitting-corrosion resistance, respectively. Simultaneous neutron diffraction analyses revealed that both samples contained additional martensite phase due to strain-induced martensite transformation. The unexpectedly high martensite content (46.20 vol%) in G&H was caused by combination of lower austenite stability (CF = 17.37, p = .03), excessive cold working and inadequate thermal treatment during material processing. Together, those results assist in revealing alloying recipes and processing history, and relating these with corrosion resistance and mechanical properties. The present methodology has allowed access to unprecedented length-scale (μm to sub-nm) resolution, accessing nano- through meso-scopic properties. It is envisaged that such an approach can be extended to the study and design of other metallic (bio)materials used in medical sciences, dentistry and beyond.

Catalyst-Free, High-Loading Silicon Nanowires Anodes for Lithium Ion Batteries

We report on the scalable synthesis and characterization of novel architecture three-dimensional high-capacity amorphous Silicon Nanowires (SiNWs)-based anodes, with less than a third of the thickness of common graphite anodes. The SiNWs were grown by a novel, catalyst-free chemical vapor deposition (CVD) process, on a commercial stainless steel (SS) mesh. By using our novel, low-cost CVD procedure we synthesized binder-free anodes with loadings of up to 5mgSi/cm2, that have shown stable cycle life for over 600 cycles and provided capacities of up to 6mAh/cm2, very low (<10%) irreversible capacity and good compatibility with commercial cathodes. With the use of two- and three-electrodes coin cells we focus on studying their electrochemical performance and degradation mechanisms. It was found that the observed capacity loss of the SiNWs-based anodes is mainly caused by the increase in the resistivity of the SEI layer, along with the steady rise of the charge/discharge overpotential detected on both the anode and the cathode. Our anodes have been coupled with commercial cathodes (LFP and NCA), and have the potential to increase the energy density of LIBs by over 40%. In order to prove the scalability of our anodes, they have been successfully incorporated in industrial-size cylindrical cells, proving their potential as a viable candidate for anodes of the next generation lithium ion batteries for portable electronics and electric vehicles. Most recently, these anodes have been successfully coated with solid polymer electrolytes and cycled in all-solid state cells as a proof of concept, paving the way for safer, more durable LIBs.

Plastic anisotropy and deformation-induced phase transformation of additive manufactured stainless steel

Plastic anisotropy and deformation-induced phase transformation of additively manufactured (AM) stainless steels were investigated via in-situ neutron diffraction, electron backscatter diffraction, metallography, and fractography. Two types of tensile specimens were manufactured: (1) One sample was vertically fabricated with its tensile axis parallel to the z-direction (AM-V), (2) The other sample was horizontally fabricated with its tensile axis perpendicular to the z-direction (AM-H). A commercial 15-5PH stainless steel (CA) was used for comparison. AM steel revealed enhanced yield strength, tensile strength, and uniform elongation over CA, which was mainly due to grain refinement and transformation induced plasticity (TRIP). Different onsets of strain nonlinearity between AM-V and AM-H were closely related to martensitic phase transformation. Stresses estimated from lattice strains measured by neutron diffraction matched well with the applied stress-strain curves. After plastic deformation, voids were formed and congregated near the solidified line where fine grains were populated. Higher dislocation density was observed in the fine grain zone, and lower density was shown in the relatively coarse grain zone. AM steels exhibited significant anisotropic fracture behavior in terms of loading direction. In contrast to isotropic failure for CA and AM-V, AM-H revealed anisotropic failure with elliptical formation of the fracture feature. The fracture surface of AM-H possessed many secondary cracks propagating perpendicular to the building direction. The occurrence of secondary cracks in AM-H resulted in rapid load drop during tensile loading after necking.

Corrosion inhibition of stainless steel in sulfuric acid solution containing sulfide ions

PurposeAlthough stainless steel (SS) has good corrosion resistance in most aqueous solutions, it suffers corrosion in some solutions which contain aggressive ions such as sulfide ions. This study aims to use some cephalosporins (cefotaxime, cephapirin and cefazolin) as corrosion inhibitors of commercial SS in 0.5 M H2SO4solution containing sulfide ions at 30°C.Design/methodology/approachThe study was carried out using weight loss method, potential-time, linear polarization, potentiodynamic polarization, electrochemical impedance measurements, scanning electron microscopy, Fourier transform infrared and energy dispersive X-ray analysis.FindingsThe presence of the cephalosporin compound in the corrosive medium shifted the corrosion potential of SS to much positive side, which enhances self-passivation of SS, and the shifting increased with increasing inhibitor concentration. The cephalosporin compounds worked as effective inhibitors with mainly anodic and the efficiency increase as cefotaxime &lt; cephapirin &lt; cefazolin. The inhibitors form a protective adsorbed layer, which enriches the surface content of Ni and Cr and thus assists the SS to be passive.Originality/valueThe antibiotics cephalosporins could be used as effective corrosion inhibitors for SS in acidic solutions containing sulfide ions. The inhibitors enhances the the passive oxide film of SS even in presence of aggressive ions such as sulfide ions.

Microstructure, strain-rate sensitivity, work hardening, and fracture behavior of laser additive manufactured austenitic and martensitic stainless steel structures

The tensile flow properties of austenitic (S316-L) and martensitic (S410-L) stainless steel wall structures deposited by powder-fed laser additive manufacturing (LAM) process are evaluated. The properties obtained by the LAM process are compared to commercial rolled sheets of these metals. Strain-rate sensitivity, work hardening, and fracture behavior are assessed by conducting uniaxial tensile testing at different strain rates (0.001, 0.01, 0.1, and 1.0 sec−1). Moreover, a correlation between the final microstructure and mechanical properties is established for the LAM products through detailed analyses of grain structures and hardness indentation measurements. The results indicate a strong dependency for the strain rate in martensitic alloys compared to austenitic alloys produced by the LAM process. Interestingly, the tensile strength of commercial rolled martensitic stainless steel sheet doubles (∼100% increase) by increasing the strain rate, while preserving its elongation to failure. Comparing the two manufacturing methods, a lower strain-rate sensitivity factor is recorded for the additive manufacturing material (m of ∼0.0336) compared to the commercial sheet (m of ∼0.0775). This lower sensitivity is attributed to coarser grain structure and greater microstructural heterogeneity of the LAM product, which stems from directional solidification and cooling phenomenon during the layer-upon-layer deposition process. In contrast, the work hardening exponent (n value) varies little (0.1834–0.2854) for the different materials and manufacturing methods. Fractographic studies reveal that the fracture mode varies from ductile rupture towards ductile-brittle with the formation of greater martensitic phases, which is in combination with the failure component changing from shear to tensile at high strain rates.

On the microstructure effects when using electropulsing versus furnace treatments while drawing inox 308L

In the present work, microstructure and texture evolution in a commercial 308L stainless steel wire is assessed by comparing several electropulsing configurations and thermal annealing treatments performed during and after a single pass wire drawing process. In this context, the objective is to determine if the electropulsing brings significant differences on the material microstructure with respect to a conventional annealing treatment. To this end, electron backscattered diffraction was used to analyze the microstructure changes in all the studied cases. The results indicate that recrystallization and detwinning are drastically accelerated when in situ electropulsing treatment is applied in the specimen during its drawing. Moreover, electropulsing or large times of furnace treatments after the wire drawing are found to enhance the material conductivity compared to the in situ electropulsing treatment during the wire drawing. Finally, differential scanning calorimetry and material resistivity are performed to analyze the thermal events in a meso/micro scale. Accordingly, the bulk temperature for the electropulsed specimens is assumed to be much higher than the bulk temperature recorded at the surface of the specimen.

Effect of The Degree of Cold Work and Sensitization Time on Intergranular Corrosion Behavior in Austenitic Stainless Steel

Abstract Present paper deals with the influence of a wide range of cold rolling (5, 10, 15 and maximum 40% cold deformation) and the sensitization time (aging at 700°C for 0.12, 0.5, 1, 4, 16 and 32 hours) on intergranular corrosion (IGC). Intergranular corrosion of commercial stainless steel type X6CrNiTi18-10 (1.4541, AISI 321) is frequently observed in several process environments. These localized attacks are normally attributed to the carbide precipitation and concomitant depletion of chromium near grain boundary due to steel exposure to sensitization temperature. Such undesirable microchemistry is expected to be changed further if the material undergoes deformation prior to sensitization. The consequences of deformation on IGC have been investigated by using EN ISO 3651-1methods (Huey test – Corrosion test in nitric acid medium by measurement of loss in mass). Introducing deformation to the investigated stainless steel seems to change the kinetics of carbide precipitation M23C6 and thereby changes it resistance to IGC. Cold deformation before sensitization reduces the intensity of intergranular corrosion of this steel. The deformed structure created during the cold work process, numerous slip planes and the twins boundaries are just like the grain boundaries and the places where the chromium carbides preferentially precipitates. Due to the more evenly occurring precipitation processes within the whole deformed grains, there is no phenomenon of local grain boundary carbide precipitation, and thus there is no decrease in the resistance of this steel to intergranular corrosion. The assessment of the degree of intergranular corrosion was based on the measurement of mass loss and observation of corroded surfaces on optical and electron transmission and scanning microscopes.

In-situ evolution of active layers on commercial stainless steel for stable water splitting

Efforts to explore earth-abundant, non-precious electrocatalyst, especially commercial stainless steel, to replace precious-metal-based catalyst have attracted increasing interest in renewable energy research. Herein, we design a facile and simple route to fabricate highly efficient 316L-type stainless steel-based electrocatalysts for water splitting by CH4 plasma. After CH4 plasma treatment, the amorphous carbon layer and the graphene encapsulated Fe3C nanoparticles are observed on the surface of stainless steel, which play the roles of active sites and protective layer for simultaneously providing an acceptable hydrogen evolution reaction (HER) and excellent oxygen evolution reaction (OER). The optimized stainless steel-based electrocatalyst exhibits an overpotential of only 290 mV at 10 mA cm−2 and possesses outstanding kinetics (the Tafel slope of 38 mV dec-1) for OER in the 1.0 M KOH aqueous solution. We anticipate that the operating strategy of our system may aid the development of commercial non-precious productions as the efficient electrocatalysts for energy storage and conversion.

Novel experimental method to determine the limit strain by means of thickness variation

In this work an experimental method to determine the limit strain by means of digital image correlation is proposed. This method analyzes the variation of the thickness of a sheet metal through the temporal evolution of the strains, assuming incompressibility during the plastic strain process. To achieve this objective, the center and the edge of the necking area are identified with the strain history and the strain rate, respectively. The limit strain is thus determined when a non-homogeneous decrease in the thickness necking area begins, in contrast with existing methods based on performing analysis of surface strains. The proposed method is applied to determine the forming limit curve of commercial stainless steel and compared with other approaches. The results indicate that the proposed methodology is more reliable for determining the forming limit curve of the sheet, because it captures the heterogeneity of the strain distribution at the local necking site. The microstructure and the textures of the initial material are characterized via optical microscopy, x-ray diffraction and electron backscattering diffraction. The mechanical properties of the material are discussed in the light of the microstructural analysis.

Highly conductive and stable Mn1.35Co1.35Cu0.2Y0.1O4 spinel protective coating on commercial ferritic stainless steels for intermediate-temperature solid oxide fuel cell interconnect applications

Chromia scale growth and Cr evaporation of ferritic stainless steel interconnects are known to be major causes of serious degradation of the solid oxide fuel cell (SOFC) stack. The development of suitable ceramic coating materials on the metallic interconnects has been demonstrated as an effective way to address these challenges. Herein, we developed a Mn1.35Co1.35Cu0.2Y0.1O4 (MCCY) spinel material via a facile glycine-nitrate process as a protective coating on a metallic interconnect (SUS 441). Crystal structure and surface charge state analysis of the MCCY material revealed that co-doping of Y and Cu into the (Mn,Co)3O4 spinel resulted in redistribution of the Mn ions (Mn3+ and Mn4+) into the octahedral site, which increased the electrical conduction by enhanced small polaron hopping. Accordingly, the MCCuY-coated interconnect exhibited ∼8 times lower area specific resistance (ASR) than that of the undoped Mn1.5Co1.5O4 (MCO) coated interconnect. Moreover, time-dependent ASR behavior of MCCuY-coated sample was monitored in-situ using electrochemical impedance spectroscopy at 650 °C, showing excellent stability with no observable change for >1000 h, while the ASR of the MCO-coated sample was raised by ∼71%. After 1000 h operation, we found strong adhesion between the MCCuY coating and the metallic interconnect as well as remarkably restricted Cr diffusion into the coating layer. Furthermore, the parabolic constant associated with the oxidation kinetics of the MCCuY-coated substrate (8.25 × 10−11 mg2 cm−4 s−1) was ∼1 order of magnitude lower than that of the MCO-coated one (7.34× 10−10 mg2 cm−4 s−1) at 650 °C after 1000 h measurement. These results demonstrate that the MCCuY is a highly promising coating material of metallic interconnects for intermediate-temperature SOFC applications.

In situ TiN-reinforced CoCr2FeNiTi0.5 high-entropy alloy composite coating fabricated by laser cladding

The fcc structural CoCr2FeNiTi0.5 high-entropy alloy (HEA) composite coating with TiN particles reinforced was acquired by laser cladding on the commercial 904L stainless steels. The results show that phase structure is mainly composed of fcc solid solution and TiN phases. The coating exhibits excellent structural stability below 850 °C. The microstructure consists of irregular dendrite and TiN particles. Transmission electron microscopy (TEM) results reveal that the close-packed plane of fcc phase is (111) with interplanar spacing of ~ 0.208 nm. The interface between TiN and fcc matrix is semi-coherent. And the angle of boundary between dendrite and matrix is ~ 65°. The hardness and corrosion resistance of coating have much improvement compared with those of substrate.

Study of behavior alloy Ti and 316L in to simulated body fluid by electrochemical techniques

In this work, Ti-316L stainless steel was produced, and its electrochemical behavior was characterized. Tisteel alloys were produced using an induction furnace, by mixing commercial Ti and 316L stainless steel. Xray diffraction analyses showed the presence of both the materials, Ti and 316L, in the samples produced. The elemental composition of the materials was determined by optical emission spectroscopy and energydispersive spectroscopy, which showed similar quantities of both the elements. Both commercial 316L stainless steel and Ti–steel were studied in simulated biological fluid for imitate a like composition of body blood plasma. Electrochemical experiments conducted at 37°C indicated the stable passive polarization behavior of the Ti-steel alloy. Furthermore, the electrochemical behavior of the 316L stainless steel was also analyzed for comparison purposes. Corrosion velocities were determined using the Tafel method and corrosion resistances were obtained using the electrochemical impedance spectroscopy. The Ti-steel exhibited better protection capacity, compared with the commercial 316L steel. The corrosion velocity and the passive current density of the Ti–steel alloy were lower than those exhibited by the 316L stainless steel. Keywords: XRD, elemental composition, Tafel curves, corrosion velocity.

Influence of ageing on thermomechanical fatigue of austenitic stainless steels

The increasing demands on efficiency and flexibility results in more detrimental operating conditions for the materials used in the boiler section of biomass power plants. Hence, the materials will be exposed to cyclic operation, increased temperature and higher pressure where austenitic stainless steels have shown to be promising candidates to cope with the increasing demands. However, their behaviour under these conditions are not yet fully understood. This work investigates the response of the commercial austenitic stainless steels Sanicro 25, Sanicro 31HT and Esshete 1250 and focus on the material degradation from cyclic operating condition and long-term usage. Pre-aged specimens (2000 hours at 800 °C) were exposed to thermomechanical fatigue (TMF) testing under strain-control and in the temperature range of 100 °C to 800 °C. The two TMF-test conditions IP- and OP-cycling performed on the investigated materials did not differ that much in hardening behaviour. However, the plastic strain range were in general greater for IP-cycle condition. An observed barrelling effect was simulated by a simplified finite element analysis (FEA), which showed influence of specimen geometry. The degree of deformation of the test specimens was measured by the use of low angle grain boundary (LAGB) density calculations and it showed no barrelling effect before 500 OP-cycles.

In situ evolution of the active phase on stainless steel mesh toward a cost-effective bifunctional electrode for energy storage and conversion.

Developing low-cost, efficient and stable electrode materials is a major challenge of energy storage and conversion. Here, we report a facile, cost-effective and scaled-up self-sacrificing strategy for transforming commercial stainless steel into highly active and ultrastable electrodes for supercapacitors and the hydrogen evolution reaction. The modified stainless steel displays superior electrochemical activity as well as excellent cycling durability.

Study on the fracture behavior of the planar-type solid oxide fuel cells

In this work, the commercial SUS430 ferritic stainless steel was used as the current collector to study the fracture behaviors of the planar electrolyte-supported and anode-supported solid oxide fuel cells with the size of 10 cm × 10 cm during the thermal cycle process. The anode-supported cells (ASCs) were fabricated by tape-casting, spraying, screen-printing and co-sintering. The electrolyte-supported cells (ESCs) using fully-stabilized zirconia as material were prepared by tape-casting, multilayer lamination, screen-printing and co-sintering. The output power densities at 0.7 V (P0.7V) of the anode-supported and electrolyte-supported cells could reach 0.60 W cm−2 and 0.43 W cm−2 at 850 °C, respectively. During the thermal cycle testing, the ESC sample was cracked, while the ASC sample had no obvious changes. The thermal expansion coefficient (TEC) of the different samples was studied, and the two cells were characterized by the XRD after the thermal cycle testing. In addition, the fracture behaviors of the two cells were discussed. The results indicated that the ASC sample had the good stability during the thermal cycle testing. The excellent anti-cracking performance of the ASC was attributable to two aspects. Firstly, the TEC of the anode support is more close to that of the SUS430 material; secondly, the phase transformation of zirconia from monoclinic phase to tetragonal phase induced by stress is favorable for contraction of the anode support layer during the cooling process.

Transparent Copper-Based Antibacterial Coatings with Enhanced Efficacy against Pseudomonas aeruginosa.

Bacterial surface contamination is a major cause of hospital-associated infections. Antibacterial coatings can play an important role in reducing bacterial transmission via inanimate surfaces in healthcare settings. In this work, transparent copper-based antibacterial coatings were fabricated on commercial poly(vinyl fluoride) and stainless steel. Acrylated quaternized chitosan and ethylenediaminetetraacetic acid were covalently grafted on the substrate for complexation with copper ions. The number of viable Staphylococcus aureus in a droplet [containing ∼104 colony forming units (CFU)], deposited on the copper-containing coating decreased by ∼96% within 60 min at 25 °C. With Pseudomonas aeruginosa, one of the most virulent and hardest to kill bacteria, no CFU could be observed within the same time span (killing efficacy >99.8% based on the detection limit). An increase in copper release from the coating was observed in the presence of P. aeruginosa, which was postulated to be due to the proteolytic activity of P. aeruginosa. The higher efficacy of the coating against P. aeruginosa compared to S. aureus is thus attributed to this increased copper release from the coating, which resulted in extensive bacterial membrane damage and death. The copper-containing coating on poly(vinyl fluoride) retained its antibacterial efficacy after 100 wipes with a water-wetted cloth or isopropanol wipes, demonstrating its durability and long-term efficacy. The coating did not exhibit significant cytotoxicity toward mammalian fibroblasts, further demonstrating its potential for mitigating bacterial transmission in a clinical setting.

Ultrashort pulsed laser ablation of stainless steels

Abstract This research investigated the ablation process of commercial stainless steels with ultrashort pulsed laser. Square hollows were ablated on stainless steel sheets using a picosecond laser with pulse duration of 0.25, 1 and 10 ps, respectively, and fluence ranging from 0.125 J/cm2 to 5 J/cm2. For each setting the surface quality and ablation efficiency were determined by optical microscopy and scanning electron microscopy (SEM). Processing windows producing high quality surfaces at high ablation efficiency were found at a fluence around 0.75 J/cm2 for the two shorter pulse lengths tested. Individual cones and periodic cone-like protrusions were found in the low fluence regime ( 0.875 J/cm2), respectively, both of which make the ablated surface rough. Emphasizing on the individual cones, it is found that the cones are caused by inclusions in the base materials, attributed to a higher fluence threshold required for ceramic-like inclusions. This novel theory explaining the creation mechanism is verified by multiple analysis methods like SEM and energy dispersive X-ray spectroscopy and stop-motion SEM imaging.