Shen-Chih Lee

Effect of substrate surface roughness on the characteristics of CrN hard film

Abstract Specimens of high speed steel were coated with CrN hard film by the random arc physical vapor deposition process. The effect of surface roughness of substrates on the corrosion behavior was examined by immersion and anodic polarization corrosion tests. High temperature heating of test coupons was executed to study oxidation resistance of the chromium nitride film. Wear and adhesion behaviors were evaluated as well. The results revealed that a smooth substrate surface not only provided better adhesion as well as lower friction coefficient of the coated specimen but also improved its corrosion resistance. Wear resistance of coated material was also improved as compared with uncoated material. Oxidation and conversion of CrN to Cr 2 O 3 started at a temperature of about 700 °C, where film hardness deteriorated.

Study of high cycle fatigue of PVD surface-modified austempered ductile iron

Austempered ductile iron (ADI) is made from ductile iron by an austempering treatment, and its main microstructure is ausferrite that is composed of acicular ferrite and high carbon austenite. The purpose of this experiment is to investigate the influence of different coating layers and the size of casting (mass effect) on the high-cycle fatigue properties of ADI. Specimens in two casting sizes of the same chemical composition were subjected to a high-toughness austempering treatment, then coated with TiN or TiCN hard films by a physical vapor deposition (PVD) process. The results showed that the fatigue limit of the small casting size ADI is 292 MPa for ADI coated with TiN and 306 MPa for ADI coated with TiCN, which are 16% and 22%, respectively, higher than that of the ADI without coating (251 MPa). For the large casting size ADI, the fatigue limits are 200, 214 and 217 MPa for ADI without coating, ADI coated with TiN and ADI coated with TiCN, respectively. ADI coated with TiN and with TiCN are 7% and 9% better than the uncoated. Thus, it is concluded that TiN and TiCN coatings by PVD can improve the high-cycle fatigue strength of ADI. This is due to the high surface hardness and possibly the ADI surface compressive residual stress as well. For the small casting size ADI, TiCN-coated specimens have a bit higher fatigue strengths and this might be attributed to the higher hardness of TiCN than TiN films. As to the effect of mass, it is found that the small casting size has better fatigue properties and benefits more from the coating films. This could have stemmed from the higher nodule count and its associated benefits in thinner castings.

Effects of titanium addition and section size on microstructure and mechanical properties of compacted graphite cast iron

Abstract Titanium is an anti-spheroidizing element and also carbide former in ductile iron. On the other hand, increasing the casting size essentially lowers the cooling rate that opposes the chilling tendency of titanium. This research was to study the combined effects of titanium and section size on their promotion of compacted graphite (CG) formation, and at the same time how matrix constituents were altered in heavy-wall castings. It was found that at the increasing casting thickness of 30 mm, 65 mm and 80 mm, the percentage of CG increased while that of pearlite decreased either with or without titanium addition. However, titanium (added in an amount of 0.15 wt%) effectively promoted the formation of CG by over 10% and at the same time increased the pearlite content in the matrix. This was especially true in the thinner 30 mm casting. Irons with titanium addition exhibited a bit lower Brinell hardness, elongation, and impact toughness due probably to the higher CG percentage that facilitated easier crack propagation. However, comparing to the un-alloyed iron, fracture toughness increased along with tensile strength for iron with titanium addition in all casting sizes of 30–80mm. The higher pearlite content in the matrix has overridden the effect of increased CG percentage such that tensile strength and KIC value both increased.

Ultrasonic Nondestructive Evaluation of Matrix Structures and Nodularity in Cast Irons

The primary purpose of this research was to investigate the nondestructive ultrasonic wave response, in terms of acoustic velocities and attenuation of sound energy, in cast irons with different nodularities and matrix structures and its correlation with mechanical properties. The results indicated that the influences of matrix structures on the acoustic velocities were not apparent in the cast irons investigated. As to the nodularity, when graphites were largely spheroidal in shape (i. e., nodularity over 80 pct), the velocity of longitudinal waves propagation was about 5300 to 5500 m/s. The velocities seemed to decrease linearly down to nodularity of 25 pct, where velocity was approximately 4800 m/s. Below 25 pct nodularity, the values of acoustic velocity dropped rapidly to about 4000 to 4200 m/s. This represented the velocity of longitudinal waves propagation in gray cast iron, in which the graphites appeared in flake form. The analysis of the attenuation of ultrasonic amplitude indicated that when the nodularity of cast irons is low, the echo sound amplitude will decay more rapidly with respect to distance of echo sound travel. As to the matrix structures, ferritic, bainitic, ferritic-pearlitic (low pearlite content) and tempered martensitic matrix structures were found to have similar ultrasonic attenuation characteristics at the testing frequency of 2 MHz. A higher amount of pearlite (over 90 pct) or fresh martensite in the matrix of cast irons has resulted in faster attenuation of ultrasonic energy, with the fresh martensitic matrix being the fastest. At a testing frequency of 4 MHz, the attenuation of the ultrasonic amplitude in pearlitic and fresh martensitic matrices was found to be even greater than that of 2 MHz. However, other matrices exhibited similar attenuation behavior at both 2 and 4 MHz frequencies. The relationship between the mechanical properties of various cast irons and ultrasonic characteristics was also examined.

The effects of surface hardening on fracture toughness of carburized steel

This research program was carried out to evaluate the effects of surface hardening on the fracture toughness of carburized steel. The materials AISI 8620 steel was machined into compact-tension (CT) specimens. The specimens were pack carburized at 930°C (1706°F) for different periods of time, cooled to ambient temperature and subsequently tempered at various temperatures for one hour. The fractured specimens were examined by hardness tests, metallography, X-ray diffraction analysis for retained austenite in the case, and scanning electron microscope fractographic analysis of the fracture surfaces. The experimental results revealed that theKIC values of the carburized, AISI 8620 steels were improved by the increase in case depth. Martensitic/tempered-martensitic structure in the case was the major constituent contributing to the improved toughness. The amount of retained austenite at the case increased as the thickness of the hardened layer increased. But retained austenite as well as large grain size were found to have adverse effects on fracture toughness of the carburized steel. The tempering temperature of 500°C (932°F) provided maximumKIC values. Higher tempering temperatures resulted in sharp decrease of fracture, toughness values.

Thermal fracture endurance of cast irons with application study of pig iron ingot molds

Pig iron ingot molds manufactured with flake, compacted graphite cast iron, and spheroidal graphite cast iron were installed on a pig iron casting machine and subjected to thermal cycling for studying thermal fracture endurance of the three cast irons. The effects of graphite morphology on the fracture mechanism were analyzed by examining the fracture patterns, microstructures, and microcracks in the failed molds. The determining factors of thermal fracture endurance were elucidated with thermal fracture resistance indices. Compacted graphite cast iron exhibited better thermal fracture endurance than flake and spheroidal graphite cast irons because of its higher strength-to-thermal stress ratio.

Hydrogen transport and fracture toughness of case hardened steel

Abstract The purpose of this study is to assess the effects of effective case depth (ECD) and tempering temperature on hydrogen transport and fracture toughness of carburized AISI 8620 alloy steel. The material was machined into thin discs for permeation and into compact-tension specimens for fracture toughness measurements. The specimens were pack carburized at 930 °C and cooled to ambient temperature. The carburized specimens were austenitized at 840 °C in a high temperature salt bath, then oil quenched and tempered at various temperatures for one hour. Assessment of hydrogen transport was conducted by the electrochemical permeation technique. Both permeability and effective diffusivity decrease as ECD increases and tempering temperature decreases. Fracture toughness of pre-charged carburized 8620 steel increased with depth of carburization. Fracture toughness of hydrogen pre-charged carburized 8620 steel was minimum at tempering temperatures between about 150–400 °C. The results indicated good empirical correlation between fracture toughness and apparent solubility at higher ECD and tempering temperature. While this study shows that correlations exist, there are compositional and microstructural factors to be sorted out before any definitive relationship can be established.