Stainless Steel Forgings Research Articles

Numerical simulation and experimental validation of microstructure evolution during the upsetting process of a large size martensitic stainless steel forging

The microstructure evolution, plastic deformation, and damage severity during the open die hot forging of a martensitic stainless steel were investigated using finite element (FE) simulation. A microstructure evolution model was developed and combined with a visco-elastoplastic model to predict the strain, the strain rate, and the temperature distribution, as well as the volume fraction and the size of dynamically recrystallized grains over the entire volume of an industrial size forging. The propensity to damage during hot forging was also evaluated using the Cockcroft & Latham model. The three models were implemented in the FE code and the results analyzed in terms of microstructure inhomogeneity and stress levels in different regions of the forging. A good agreement was obtained between the predicted and the experimental results, demonstrating that the simulation provided a realistic representation of the forging process at the industrial scale.

Establishing the hot working window for the forging of S32906 super duplex stainless steel through considering deformation parameter sensitivities

The evaluation of the hot workability of the super duplex stainless steel (SDSS) material still lacks scientific and theoretical basis, leading to the frequent occurrence of unacceptable defects in its forgings. In this study, a novel method for estimating the hot workability of S32906 SDSS was proposed and verified through forging. Firstly, the flow behavior and microstructural characteristics were demonstrated by conducting isothermal compression tests at the deformation temperature from 1000 °C to 1250 °C and the strain rate from 0.01 s−1 to 10 s−1. The influence of microstructure on flow and work hardening behaviors was correlated and displayed. Then, based on the deformation characteristics presented through the variations of strain rate sensitivity (SRS) and deformation temperature sensitivity (DTS) coefficients, the SRS and DTS maps were constructed and overlaid for obtaining the hot working window. Ultimately, a large plate type forging of S32906 SDSS applied in engineering was used to verify the applicability of the obtained window. The new method can provide an in-depth understanding of hot workability and enable control of the forming and microstructural properties for this unique material.

Experimental characteristics of 316L stainless steel thin wall processed in wire-based arc additive manufacturing process

Wire arc additive manufacturing (WAAM) is an innovative technology for the fabrication of larger metal parts with complex geometry. It has a higher deposition rate, low wastage, production efficiency, and close tolerance. In this work, 316L stainless steel parts were manufactured by cold metal transfer-based WAAM process, and the effects of microstructure and mechanical properties were investigated. Microstructure studies reveal the formation of columnar and equiaxed grain morphologies with the development of epitaxial growth along the building direction. The radiographic test confirms various defects such as pores and cracks associated with the 316L deposited part. However, the microhardness studies show a uniform distribution of hardness values. The average value of microhardness of the as-deposited part was 220 HV. Tensile studies show the horizontal samples were ultimate tensile stress (UTS): 576 MPa, yield stress (YS): 302 MPa, and elongation (EL): 35% in comparison with the vertical samples were UTS: 469 MPa, YS: 295 MPa and EL: 39% and the sample exhibits the anisotropy properties. Further, the fracture surface of the tensile sample exhibits a ductile fracture with enough dimples, voids, and plastic deformation. The result shows that fabricated WAAM stainless steel 316L parts exhibit the finer microstructure and improved mechanical properties, which is extremely better than 316L stainless steel casting, forging, and wrought counterparts.

Interfacial heat transfer coefficients in cold forging of stainless steel

In numerical simulations of cold forging processes, the heat transfer coefficient has a significant influence on the resulting temperature fields. However, the values determined so far for steel materials are not in the load range of cold forging processes. In this paper, a test stand for the determination of the interfacial heat transfer coefficient under typical loads of metal forming processes is presented. The results are discussed at an example of a cold extrusion forming process. It can be shown that the value increases significantly with increasing contact pressures and are higher compared to values known from literature. The investigations also disclose the influences of lubricants and the mechanical surface treatment on the heat transfer coefficient.

Particle deformation and microstructural evolution of high borated stainless steel consolidated by Gleeble-simulated direct powder forging

ABSTRACT In the present work, a novel method was first proposed to realise direct powder forging of high borated stainless steel on Gleeble through hot compression tests performed at 1150°C and 0.01–10 s−1 with strains of 32% and 50%. Particle deformation and microstructural evolution of high borated stainless steel powder during direct forging were investigated. Backscattered electron (BSE) and electron backscatter diffraction (EBSD) were employed for microstructure examination. Microstructure reveals that large deformation zones were easily distinguished by boride and grain features. Enhanced boride coarsening in the contact area of powders was attributed to the combined action of strain, dislocation, stress and temperature rising. Strain rates had no obvious influence on densification but significantly affected the deformation of particles. Furthermore, the mechanism of particle deformation enhanced by a higher strain rate was discussed.

Process optimization for hot forging of difficult parts by computer experiments and response surface analysis

Closed die hot forging of 304L stainless steel is difficult compared to carbon and micro-alloyed steels in terms of higher forming force and defects. An appropriate combination of design and process factors including cross-section diameter, initial billet temperature, and flash land thickness is crucial to minimize the forming force without causing any under-filling or folding defects. This paper aims to develop an optimized closed-die hot forging process for long, thin, and tubular rail body components used in gasoline direct injection systems. The effects on the forging load are investigated using computer experiments containing the finite element method under a full factorial array of three factors at three levels to obtain the main and interaction effects. Linear and quadratic surface response analyses are performed to define the relation between forging load and input factors, and finally, the simulations are validated with some limited experiments. The simulation results that fit the nonlinear regression model were in close agreement with the experiments, and an optimal set of parameters is conveniently proposed for industrial production.

Microstructure and Properties of 304 Stainless Steel Specimen Manufactured by Cold Metal Transfer + Pulse Composite Arc Additive

304 stainless steel test block was fabricated by continuous melting wire with CMT and pulse mixed mode, and the path of additive manufacturing is layered slice S-shaped. The relationship between microstructure and properties of the specimen was investigated by microscope, SEM, EBSD, XRD, tensile, impact and electrochemical experiments. The results show that molding between weld and weld is very good, and the microstructure is mainly Austenite, Ferrite and a little of σ, and there are three kinds of Ferrite morphology: cellular, wormlike and lath. σ phase precipitates easily in regions with high ferrite content and is distributed at the boundary between austenite and ferrite. The specimen has good low temperature toughness. The microscopic fracture surface is mainly dimple, and the precipitates in the fracture surface are mainly fine carbide particles. The tensile strength of the additive manufacturing 304 specimen is higher than the forged specimen, and the type of fracture is ductile fracture. The electrochemical analysis of 304 stainless steel specimens and forgings shows that CMT and pulse arc additive manufacturing specimen has excellent corrosion resistance and its corrosion current is slightly lower than the forging.

Evolution of interfacial characteristics and mechanical properties for 316LN stainless steel joints manufactured by hot-compression bonding

Abstract In this study, we report a method to manufacture heavy stainless steel forgings using a newly-developed multilayer hot-compression bonding method. In particular, the interfacial healing mechanism and mechanical properties of hot-compression bonding joints under various deformation and holding conditions were systematically investigated. The results indicate that the bonding interface can be hardly healed when holding at 900 °C and 1000 °C since the interfacial oxides are difficult to decompose at the low temperatures, resulting in poor tensile properties of the bonding joints. As holding temperature increases to 1100 °C, the interfacial oxides are partially decomposed, and therefore the interface bonding effect improves with the partial restoration of tensile properties. Especially, after holding at a higher temperature, i.e., 1200 °C for 18 h, the tensile properties of the bonding joint with deformation strain of up to 35 % can be fully recovered to the level of the base material, owing to the complete decomposition of broken interfacial oxides. Based on the optimal deformation and holding conditions for interface bonding, a heavy plate with excellent mechanical properties was successfully manufactured using the multilayer hot-compression bonding method.

Characterisation of heterogeneous microstructure in large forged products using nonlinear ultrasonic method

ABSTRACTA comparative study between the ultrasonic attenuation and the nonlinearity is presented for the characterisation of microstructure in large dimension forgings. Results are provided for two austenitic stainless steel forgings of AISI type 304L with a diameter 200 mm, where microscopic observations reveal abnormal grain growth near the surface of one of the forgings. Frequency dependence of the nonlinearity parameter is used to discern variations in grain size in these forgings more precisely than attenuation measurements. The distribution of grain size is shown to have a significant influence on the nonlinearity parameter. Relative changes in the nonlinearity parameter with applied frequency were correlative with the microstructural variations in both the forgings.

Process Optimization on Multilayer Morphology During 316L Double-wire CMT+P Deposition Process

Cold metal transfer (CMT) has been widely used in metal additive manufacturing for its low heat input, less splashing and high efficiency. Wire feeding speed and travelling speed are important processes that affect morphology in CMT deposition. This study optimized the forming process of 30-layer stainless-steel part deposited by double-wire and double-arc CMT plus pulse (CMT+P) process, and investigated the effect of the ratio of wire feeding speed to travelling speed on deposition morphology. The results show that asynchronous arc striking and extinguishing can improve the forming. Moreover, the deposition molding is affected by the interaction of heat input and heat accumulation. With the similar ratio of wire feeding speed to travelling speed and the similar heat input, increasing the wire feeding speed can increase the heat accumulation and the width of sample, and decrease the height. The optimum process interval of wire feeding speed to travelling speed ratio and heat input is 3.9–4.2 and 70–74.8 J/mm, respectively. Although the increasing heat accumulation makes grain coarse and slight decreases mechanical property, the highest deposition rate can be up to 5.4 kg/h, when wire feeding speed and travelling speed are 5 m/min and 120 cm/min, respectively, and the tensile strength and elongation rate of which can reach the basic standard requirements for stainless-steel forgings.

Research on forming characteristic of pulsed selective laser melting

To investigate the forming characteristic and obtain a greater process control of the pulsed selective laser melting (P-SLM), the single melting point and single-track and multi-layer block experiments were conducted to investigate the influence of various process parameters on the forming process of P-SLM. A rapid method for the optimization of parameters based on molten pool characterizations is developed. Furthermore, the relationships between the two key process parameters (exposure time, point distance) and the geometric characteristics of single tracks are investigated by linear regression analysis. The research on the surface morphology and the overlapping rate of single-track experiments shows that the value of P-SLM scanning speed is determined by the diameter of single melting point. With relatively high forming efficiency, the highest value of density can be up to 99.99% when the process parameter of exposure time is 80 μs, the point distance is 48 μm, and the hatch space is 80 μm. The yield strength, tensile strength, and elongation of the best sample are 533 Mpa, 647 Mpa, and 40%, respectively, which are higher than that of 316L stainless steel forgings standard (ASTM A473-13).

Predicting microstructure and strength for AISI 304L stainless steel forgings

Specially designed billets of AISI 304L stainless steel were forged at two temperatures, 843 °C and 941 °C, and with varying transfer times. The machined 304L billets were thick near the center (51 mm) and thin near the periphery (10 mm) such that forging with a flat platen resulted in regions of high strain (center) and low strain (periphery). Multiple metallographic samples were extracted from one experimental forging for each temperature (water quenched forging with nominal transfer times). Microstructural analysis revealed that recrystallization initiated at about 1.4 plastic strain for forgings made at 843 °C and 1.1 plastic strain for 941 °C. Mechanical tests—performed using tensile bars from multiple locations from each forging—revealed that the resulting yield strength increased with strain up ~10–20% recrystallization, then decreased. Coupled thermo-mechanical simulations estimated a temperature increase of up to 162 °C in the center of the forging due to rapid deformation in the High Energy Rate Forging (HERF) press. Simulation using updated BCJ model parameters for 304L stainless steel predicted the strain distributions using the scalar value, equivalent plastic strain. The predicted trends for percent recrystallization and yield strength from this simulation (using the internal state variable BCJ model) agreed qualitatively with experimental tensile test results, but the quantitative comparison leaves room for improvement. Both the simulation and the experimental results clarify that the microstructure and strength of 304L forgings are difficult to control in a forging with excessive strain variation. It is recommended that strain variation be minimized and overall strain be kept under ~0.5 to best control the strength and grain structure of 304L stainless steel. Excessive transfer time appeared to have little effect on the forging yield strength, but air cooling and die cooling after forging indicated softening for the forgings made at 941 °C.

Repeatability of Contour Method Residual Stress Measurements for a Range of Materials, Processes, and Geometries

Abstract This article examines the precision of the contour method using five residual stress measurement repeatability studies. The test specimens evaluated include the following: an aluminum T-section, a stainless steel plate with a dissimilar metal slot-filled weld, a stainless steel forging, a titanium plate with an electron beam slot-filled weld, and a nickel disk forging. These specimens were selected to encompass a range of typical materials and residual stress distributions. Each repeatability study included contour method measurements on five to ten similar specimens. Following completion of the residual stress measurements, an analysis was performed to determine the repeatability standard deviation of each population. In general, the results of the various repeatability studies are similar. The repeatability standard deviation tends to be relatively small throughout the part interior, and there are localized regions of higher repeatability standard deviations along the part perimeter. The repeatability standard deviations over much of the cross section range from 5 MPa for the aluminum T-section to 25 MPa for the nickel disk forging. There is a strong correlation between the elastic modulus of the material and the repeatability standard deviation. These results demonstrate the precision of the contour method over a broad range of specimen geometries, materials, and stress states.

Validation of a Contour Method Single-Measurement Uncertainty Estimator

This work validates an analytical single-measurement uncertainty estimator for contour method measurement by comparing it with a first-order uncertainty estimate provided by a repeatability study. The validation was performed on five different specimen types. The specimen types cover a range of geometries, materials, and stress conditions that represent typical structural applications. The specimen types include: an aluminum T-section, a stainless steel plate with a dissimilar metal slot-filled weld, a stainless steel forging, a titanium plate with an electron beam slot-filled weld, and a nickel disk forging. For each specimen, the residual stress was measured using the contour method on replicate specimens to assess measurement precision. The uncertainty associated with each contour method measurement was also calculated using a recently published single-measurement uncertainty estimator. Comparisons were then made between the estimated uncertainty and the demonstrated measurement precision. These results show that the single-measurement analytical uncertainty estimate has good correlation with the demonstrated repeatability. The spatial distributions of estimated uncertainty were found to be similar among the conditions evaluated, with the uncertainty relatively constant in the interior and larger along the boundaries of the measurement plane.

Effects of process parameters on fragment and refinement of millimeter-grade coarse grains for 316LN steel during hot cogging

The heterogeneous mixed-grain microstructure is a common defect for the heavy forging of 316LN austenitic stainless steel. Isothermal compression experiments were performed on a Gleeble-3500 thermo-mechanical simulator to investigate the effect of process parameters on the fragment and refinement of millimeter-grade coarse grains (MCGs) during hot cogging. The experimental results indicate that the stress of MCG specimens is much larger than that of fine grain (FG) ones at 1150 °C, while the stress difference between MCG and FG samples became smaller at 1200 °C. Moreover, the MCGs can be well fragmented and refined under the condition of temperature of 1200 °C, strain rate of 0.01 s−1, and reduction rate of 50%. Meanwhile, numerical simulations were conducted to study the influences of temperature, strain and strain rate on microstructure evolution. The results of experiments and simulations comprehensively demonstrate that the MCG results in the increase of deformation resistance and incompatibility of deformation, and it can be fragmented and refined at 1200 °C so that the plastic deformation energy decreases remarkably with the increase of temperature from 1150 to 1200 °C.

Application of Powder Metallurgy in Production of Biomaterials

Metallic biomaterials are currently produced mainly by conventional metallurgical processes, i.e. the melting and casting used e.g. in production of cobalt alloy implants, or forming processes as cold or hot working (rolling or forging of stainless steel for surgical applications). Such processes including melting are used also in production of smart biomaterials - NiTi shape memory alloys. The mechanical properties are strongly dependent on the grain size. Therefore, the techniques to obtain finer structure are very desirable to enhance the mechanical properties of the biomaterials and thus to increase lifetime of the implant. This paper is devoted to the description of the possibilites of powder metallurgy not only for the structure refinement, but also for the ptoduction of clean biomedical alloys as well as the porous bioamterials. The use of powder metallurgy is described for Co-Cr-Mo surgical alloy, Ni-Ti shape memory alloy and Ti-based porous biomaterial. In addtion to known methods, new powder metallurgy processes and materials developed by the authors are presented.

Research on the Heat Processing Theory and Damage of Martensitic Stainless Steel

Through the Gleeble-1500D thermal simulator, the paper were carried out hot compression and tensile experiments on the 1Cr13 martensitic stainless steel, and get the different deformation curve under the conditions of stress strain. At the same time, we get the constitutive equations and Z parameters, which provide necessary material model for numerical simulation of the stainless steel forging process. The compression and tensile tests were simulated with DEFORM-3D simulation software, respectively; get the material value of the fracture damage ductile fracture critical damage and material void initiation damage, it will reasonable provide reference in production process and prevention crack in the process of forging.

Status of Design and R&D for ITER Blanket in Korea

Since the decision of blanket redesign by 2007 ITER design review, the blanket system is being developed in the framework of Blanket Integrated Product Team (BIPT) composed mainly of ITER Organization (IO) and procuring parties. Korean Domestic Agency (KODA) is mainly contributing to the design and development of blanket Shield Block (SB). In particular, KODA is supporting the design activities including electromagnetic, thermo-hydraulic and thermo-mechanical analyses to complete the final design of blanket shield block. For the manufacturing of a blanket shield block conventional fabrication techniques based on drilling, milling and welding of stainless steel forged blocks have been adopted. As a consequence of the manufacturing feasibility study, key fabrication techniques to be verified beforehand have been identified and successfully developed in collaboration with related industries. The pre-qualification program of the fabrication and testing of Full Scale Prototype (FSP) is in progress. Until now the material development of 316L(N)-IG stainless steel forging has been successfully completed, and the fabrication of FSP is on-going. Even though the procurement of blanket First Wall (FW) was withdrawn at the 9th meeting of the ITER Management Advisory Committee, the participation of the 2nd pre-qualification program of EHF (Enhanced Heat Flux) FW small scale mock is being valid for securing core engineering technologies. At present the fabricated mock-ups are waiting for high heat flux test with the Electron Beam (EB) gun test facility being newly built in Korea. This paper provides the current status of design and relevant R&D activities of the blanket system to secure key technologies and to fulfill our promise to ITER project.

New challenges for the use of duplex stainless steels at low temperatures

Duplex stainless steels are often used today in the oil and gas industry because of their high strength and good corrosion resistance. New developments in arctic conditions and at high pressure require the qualification of these materials at very low temperatures. This paper will present the results of an extensive test program dedicated to assessing the low-temperature behavior of various heavy-wall duplex stainless steel forgings, hot isostatically-pressed (HIP) connectors and hot extruded pipes at temperatures down to –60 °C. Several welding processes were also evaluated. The implications for the design of duplex stainless steel production facilities will be discussed.

Nuclear engineering of diagnostic port plugs on ITER

The nuclear engineering infrastructure of port-based diagnostics on ITER is presented, including the equatorial and upper port plug generic designs, the adopted modular concept, the loads and associated load response and the remote handling. A modular approach is adopted for the internal shielding modules as this helps to reduce the dominant electromagnetic loads, reduces interfaces and accelerates remote handling. Extensive use of gun-drilling is employed to produce water channels in the stainless steel plate and forgings used to construct the port plugs and modules. The French Nuclear Code RCC-MR (2007) is employed in the design and the manufacturing. An integrated remote handling scheme is described, including a description of maintenance operations in the Hot Cell Facility.