Effects of Cold Work on Abnormal Grain Growth During Simulated Carburizing of 4121 Steel Containing Nb

Precision cold-forging processes are used to produce near-netshape parts that may then be carburized. During carburization thermal cycles, abnormal grain growth (AGG) after cold forging is known to develop microstructures which limit fatigue strength. In the present study, a small 0.04 wt.% Nb addition was made to a low-alloyed AISI 4121 steel containing 0.3 wt.% Mo. Subcritically annealed specimens were cold rolled (to simulate cold forging) at selected reduction ratios up to 50%, heated according to a simulated gas carburizing cycle at 930 °C, and water quenched to produce a final martensitic microstructure. The number density of abnormally grown grains increased rapidly as the cold rolling reduction ratio increased from 0 to 10%. With a further increase in reduction ratio, the extent of AGG decreased and was absent in samples subjected to the maximum reduction ratio of 50%. The evolution of fine (Nb, Mo)(C,N) precipitates at various stages of processing was characterized by thermodynamic calculations and electron microscopy and compared to the occurrence of abnormal austenite grain growth. The significance of these results for controlling AGG and thus optimizing fatigue performance in commercially-produced cold-forged and carburized components is discussed.

Nitriding and Ferritic Nitrocarburizing of Quenched and Tempered Steels

A physics-based software model is being developed to predict the nitriding and ferritic nitrocarburizing (FNC) performance of quenched and tempered steels with tempered martensitic microstructure. The microstructure of the nitrided and FNC steels is comprised of a white compound layer of nitrides (ε and γ’) and carbides below the surface with a hardened diffusion zone (i.e., case) that is rich in nitrogen and carbon. The composition of the compound layer is predicted using computational thermodynamics to develop alloy specific nitriding potential KN and carburizing potential KC phase diagrams. The thickness of the compound layer is predicted using parabolic kinetics. The diffusion in the tempered martensite case is modeled using diffusion with a reaction. Diffusion paths are also developed on these potential diagrams. These model predictions are compared with experimental results.

Applying the Masteralloy Concept for Manufacturing of Sinter Hardening PM Steel Grades

Sinter hardening is a powder metallurgy processing route that combines the sintering and the heat treating processes in one step by gas quenching the components immediately after they have left the high temperature zone of the furnace. It is both economically attractive and ecologically beneficial since it renders deoiling processes unnecessary. The slower cooling rates associated with gas compared to oil quenching however requires special alloy concepts different to those known from wrought steels. In the present study it is shown that by admixing atomized masteralloy powders consisting of suitable combinations of Mn, Cr, Si, Fe and C to base iron or pre-alloyed steel powders, sinter hardening PM steel grades can be produced that transform to martensitic microstructure at cooling rates of 2-3 K/s as typical for industrial sinter hardening. This is confirmed by CCT diagrams and hardness measurements. However, metallographic investigations are also necessary because in sintered steels, the cores of the largest base powder particles are alloyed very slowly during sintering and therefore tend to result in soft spots in the sinter hardened microstructure which are mostly not discernible in the CCT diagrams. Here, even slight pre-alloying of the base powder with Mo and/or Cr is helpful, both increasing the hardenability of the steels compared to base plain iron and avoiding soft spots in the microstructure.

Influence of the Increase in Aluminum Concentration on the Microstructural Behavior and Hardness of a Cu-Al Alloy (ASTM B-824)

In this work, the effect of the increase in the concentration of aluminium (Al) on the microstructure and the hardness of bronze to aluminium was studied using optical emission spectrometry, scanning electron microscopy, dispersion spectroscopy techniques of energy, conventional optical microscopy and hardness tests. Alloys with three concentrations by weight of aluminium (Cu-4.5 wt.% Al, Cu-7 wt.% Al and Cu-10 wt.% Al) were studied, with the chemical parameters established in the (ASTM B-824), were manufactured and cast in a permanent mold according to the standard (ASTM B-208), to determine the influence of the chemical concentration of aluminium the response surface methodology was used. It was observed that bronzes with a content of 4.5 wt.% Al and 7 wt.% Al, remain with a constant microstructure of phase α, and bronzes of 10 wt.% Al, undergo a change from monophasic to biphasic microstructure (α + β) evidencing the appearance of a martensitic microstructure similar to steels providing a better behaviour to resistance to indentation. It is concluded that the increase in the concentration of Al, in the Cu-Al alloy, presents a microstructural change, and the appearance of a β and β 'phase generates a better property in hardness, the hardness has a behaviour k proportional to the aluminium concentration obtaining 53 Brinell value for an increase in the concentration of 5.5 wt.% Al. Keywords: alloy, microstructure, bronze, spectrometry, spectroscopy. Resumen En el presente trabajo, se estudió el efecto del aumento de la concentración de aluminio (Al) sobre la microestructura y la dureza del bronce al aluminio, mediante el uso de espectrometría de emisión óptica de chispa, microscopia electrónica de barrido, técnicas de espectroscopia de dispersión de energía, microscopia óptica convencional y pruebas de dureza. En la investigación se estudiaron aleaciones con tres concentraciones en peso de aluminio (Cu-4,5 wt.% Al, Cu-7 wt.% Al y Cu-10 wt.% Al), con los parámetros químicos establecidos en la norma (ASTM B-824), fueron fabricadas y coladas en un molde permanente de acuerdo con la norma (ASTM B-208); para determinar la influencia de la concentración química del aluminio se usó la metodología de superficie de respuesta. Se observó que los bronces con un contenido de 4,5 wt.% Al y 7 wt.% Al, permanecen con una microestructura constante de fase α, y los bronces de 10 wt.% Al, sufren un cambio de microestructura monofásica a bifásica (α+β) evidenciando la aparición de una microestructura martensítica similar a los aceros aportando un mejor comportamiento a la resistencia a indentación. Se concluye que el aumento en la concentración de Al, en la aleación Cu-Al, presenta un cambio microestructural, y la aparición de una fase β y β’ genera una mejor propiedad en la dureza, la dureza posee un comportamiento k proporcional a la concentración de aluminio obteniendo en valor de 53 Brinell para un incremento en la concentración de 5,5 wt.% Al. Palabras claves: aleación, microestructura, bronce, espectrometría, espectroscopia.

Magnetic Methods for the Identification of Incorrect Microstructures in Grade 91 Power Station Steels

Grade 91 steels have been used in power generation for more than 20 years in high temperature, high pressure applications such as steam piping, headers and tubing because it provides superior creep and oxidation resistance at elevated temperatures. The mechanical properties of the material are dependent on the creation of a martensitic microstructure, however incorrect heat treatment during manufacture, installation or repair can result in a weak ferritic or semi-ferritic microstructure which can cause premature component failure. Currently, components with incorrect, weak microstructures are identified using hardness testing; a manual technique which is prone to error. This work details a series of tests carried out at the University of Manchester to assess the suitability of multi-parameter magnetic testing for the identification of incorrect microstructures. The tests stem from a workshop organized by the Electric Power Research Institute (EPRI) where three sets of samples (eight pipe sections, eight tube sections and eight unidentified tube sections) with different microstructures were circulated world-wide. The results of the work show that the magnetic measurement techniques employed in these tests have the potential to provide a basis for the development of a portable NDE system for the identification of incorrect microstructures in Grade 91 plant components. The developed system would enable fast scanning of components with very little surface preparation along with digital data storage, improving on current manual hardness testing.

Microstructural Characterization of Laser Weld of Hot-Stamped Al-Si Coated 22MnB5 and Modification of Weld Properties by Hybrid Welding

The presence of Al-Si coating on 22MnB5 leads to the formation of large ferritic bands in the dominantly martensitic microstructure of butt laser welds. Rapid cooling of laser weld metal is responsible for insufficient diffusion of coating elements into the steel and incomplete homogenization of weld fusion zone. The Al-rich regions promote the formation of ferritic solid solution. Soft ferritic bands cause weld joint weakening. Laser welds reached only 64% of base metal's ultimate tensile strength, and they always fractured in the fusion zone during the tensile tests. We implemented hybrid laser-TIG welding technology to reduce weld cooling rate by the addition of heat of the arc. The effect of arc current on weld microstructure and mechanical properties was investigated. Thanks to the slower cooling, the large ferritic bands were eliminated. The hybrid welds reached greater ultimate tensile strength compared to laser welds. The location of the fracture moved from the fusion zone to a tempered heat-affected zone characterized by a drop in microhardness. The minimum of microhardness was independent of heat input in this region.

Effect of Quenching Strategy and Nb-Mo Additions on Phase Transformations and Quenchability of High-Strength Boron Steels

The application of direct quenching after hot rolling of plates is being employed in the production of ultra-high-strength hot rolled plates. When heavy gauge plates are produced, the complexity involve in achieving high cooling rates in the plate core is increased and the formation of undesirable soft phases within martensite is common. In the current paper, both direct quenching and conventional quenching (DQ and CQ) processing routes were reproduced by dilatometry tests and continuous cooling transformation (CCT) diagrams were built for four different high-strength boron steels. The results indicate that the addition of Mo and Nb-Mo suppresses the ferritic region and considerably shifts the CCT diagram to lower transformation temperatures. The combination of DQ strategy and the Mo-alloying concept provides the best option to ensure hardenability and the formation of a fully martensitic microstructure, and to avoid the presence of soft phases in the center of thick plates.

Assessment of Heat-Affected Zone Softening of Hot-Press-Formed Steel over 2.0 GPa Tensile Strength with Bead-On-Plate Laser Welding

High-strength hot-press-formed (HPF) steels with a fully martensitic microstructure are being widely used in the fabrication of automotive body structure, and 2.0 GPa-strength HPF steel has recently been commercially launched. However, heat-affected zone (HAZ) softening is unavoidable in welding martensitic steel. In this study, the HAZ softening characteristic of 2.0 GPa HPF steel was investigated by applying a high-brightness laser welding process, wherein the heat input was controlled by varying the welding speed. Microstructural evaluation and hardness test results showed that the base metal with a fully martensitic microstructure was changed to the same type of fully martensitic microstructure in the weld metal, while relatively soft microstructures of tempered martensite and ferrite phase were partially formed in the intercritical HAZ (ICHAZ) and subcritical HAZ (SCHAZ) areas. In the tensile test, the joint strength was 10–20% lower than that of the base metal, and the fracture initiation was estimated at the ICHAZ/SCHAZ boundary, where the lowest hardness was confirmed by the nanoindentation technique.