Yunan Prawoto

Effect of Prior Austenite Grain Size on the Morphology and Mechanical Properties of Martensite in Medium Carbon Steel

In industrial application, unintentional manufacturing line troubles often consequence in heating raw materials excessively, in terms of either time or temperature. One of the effects of such occurrence is a product with a variation of prior austenite grain size, even if after the heat treatment the end result is the same, martensite. The variation of the prior austenite grain size is believed to vary the end results of the martensite. This undesirable variation includes the variation of fatigue resistance, impact strength, yield strength, hardness, etc. This research studies the effect of the prior austenite grain size on the morphology of the martensite microstructure. The results show that within the typical industrial application of temperature and holding time set up, as holding time or the temperature increases, the prior austenite average diameter increases. The block and packet sizes in the martensite also increase. The variation of mechanical property dependence on the grain size is indeed due to the different characteristics reflected in the martensite morphology. With respect to the same area, smaller grain has more blocks and packets, which agrees with higher dislocation density verified with transmission electron microscopic evaluation.

Carbon restoration for decarburized layer in spring steel

It is well known that decarburization has a bad effect on spring steel strength. This research presents a method to improve the product quality by means of recovering the decarburization layer. Unlike conventional methods, which usually use mechanical means, this method relies on a basic metallurgical principle, the process of diffusion. A carbon-rich layer is coated on the surface of the object. The object is then heat treated at conditions similar to the manufacturing process. To accomplish the objective, an experiment and a finite-element analysis (FEA) simulation were performed. The material chosen was a hypo-eutectoid steel with an excessive decarburization layer. The simulation was performed by digitizing the optical micrograph of decarburized raw materials and meshing the picture to get elements to start the analysis. Simple diffusion theory was then applied to the model. Various time parameters were used to simulate the redistribution of the carbon atoms. Comparable with the simulation, an experiment was also performed. The experiment began by coating a carbon-rich material onto decarburized raw material. The samples were then austenitized and subsequently either annealed or quenched. The state of the carbon restoration was then evaluated. The research concluded that the idea of carbon restoration can be implemented in the manufacturing process.

Failure Analysis and Life Assessment of Coating: The Use of Mixed Mode Stress Intensity Factors in Coating and Other Surface Engineering Life Assessment

Unlike metals, where failure analysis and life assessment methods are quite established, the failure analysis and life assessment of coatings are often underrated and disregarded. This research encourages failure analysts to realize and avail the opportunity provided by an alternative approach. The authors use energy density mechanics concepts to develop a new parameter in coating blistering. A mixed mode stress intensity factor is used as a basis for the derivation. This new parameter will be useful for the researchers and practitioners engaged in coating life assessment. It is recommended that the assessor combines field-determined adhesion strength values and blister evaluation, together with laboratory-derived strain energy density data, to quantitatively predict remaining coating life. This approach also provides a tool in failure analysis.

Coupled, macro-micro modeling for hot deformation and sintering.

Abstract Coupled, macro–micro modeling is proposed to describe hot deformation and sintering behavior of materials at the elevated temperature. Micro-model for description of microstructure evolution is coupled in the hierarchical structure with the macro-model for deformation of structural members or specimens. Owing to the homogenization theory and the selected unit-cell model, hot pressing as well as hipping processes can be quantitatively described by the present method. Direct coupling among the elasto-creep deformation, the thermal transient and the diffusion process enables us to construct the theoretical frame for quantitative description of various material behavior at the elevated temperature. Numerical examples are shown to demonstrate the validity and effectiveness of the present methodology.

Linear elastic fracture mechanics (LEFM) analysis of the effect of residual stress on fatigue crack propagation rate

In this research, both residual and applied stresses are converted to stress intensity factors independently and combined using the superposition principle. The fatigue crack propagation rates are predicted. Experiments using two different loading modes, constant applied stress intensity factor (SIF) range, and constant applied load modes are done for samples with and without initial tensile residual stresses. The samples with initial tensile residual stresses exhibit accelerations of the crack propagation rates. The results show that the weight function method combined with the three-component model provides a good prediction of fatigue crack propagation rates in tensile residual stress fields.

Coating Life Assessment: The Use of Adhesion Strength in Parametric Development in Coating Degradation Evaluation

Most of the steel structures used in industrial and non-industrial applications are exposed to outdoors weathering conditions. Organic coating typically protects them from corrosion. The maintenance actions can be done efficiently only if there is sufficient information of the condition. Therefore, the deterioration of the coating system and its lifetime has to be assessed accurately. This paper focuses on the development of parameters based on adhesion strength useful for that purpose. Three parameters are proposed, namely stress intensity factor, strain energy density, and J-integral.

Modeling creep damage based on real microstructure

This paper outlines a method to model creep failure of polycrystalline materials based on a real microstructure taken from an optical microscope. The creep failure is simulated in 304 stainless steel and the simulation is based on Norton’s creep law. By treating the grain boundaries and the grains differently and adopting the void nucleation process proposed by Shewmon, the creep strain energy density can be used as a failure criterion. The result of the simulation confirmed the results of conventional methods used in a high-temperature remnant life assessment. The intermediate results of the simulation process allow calculation/monitoring of stiffnesses degradation as the material undergoes creep failure.

Exploring the Potential Use of Adhesion Strength for Coating Analysis

This paper discusses the possibility of the potential use of adhesion strength for coating analysis. Most of the steel structures used in industrial and non-industrial applications are exposed to outdoors weathering conditions. Organic coating typically protects them from corrosion. The maintenance actions can be done efficiently only if there is sufficient information on the accurate condition of it. Therefore, the deterioration of the coating system and its lifetime has to be assessed accurately. Adhesion strength has the potential to be used as a parameter for evaluation. This paper explores the development of these parameters mainly based on fracture mechanics. transformed into a degradation rate. A time series modeling approach was proposed to predict daily degradation. Heutink et al. (6) describe how the maintenance methodology used in the Netherlands was applied to protective paint systems. The lifetime-extending maintenance model, in which deterioration is modeled by a gamma process with expected deterioration non-linear in time, is applied successfully to optimize maintenance of the coating of the Haringvliet storm-surge barrier. Among other deterioration parameters, this paper emphasizes on the adhesive strength taking the advantage of the blister formation and development as a symptom of the coating deterioration.

Stress Corrosion Cracking of Steel and Aluminum in Sodium Hydroxide: Field Failure and Laboratory Test

Through an investigation of the field failure analysis and laboratory experiment, a study on (stress corrosion cracking) SCC behavior of steel and aluminum was performed. All samples were extracted from known operating conditions from the field failures. Similar but accelerated laboratory test was subsequently conducted in such a way as to mimic the field failures. The crack depth and behavior of the SCC were then analyzed after the laboratory test and the mechanism of stress corrosion cracking was studied. The results show that for the same given stress relative to ultimate tensile strength, the susceptibility to SCC is greatly influenced by heat treatment. Furthermore, it was also concluded that when expressed relative to the (ultimate tensile strength) UTS, aluminum has similar level of SCC susceptibility to that of steel, although with respect to the same absolute value of applied stress, aluminum is more susceptible to SCC in sodium hydroxide environment than steel.