Mechanical Engineering

Cyclic heat treatment consisting of repeated short duration (6 minutes) holding above the Ac3 temperature
(850 ºC) followed by forced air cooling was preformed to C45 steel. Reflected light microscope (RLM), scanning electron
microscope (SEM), hardness measurement, and tensile testing were used to investigate the effect of this technique of heat
treatment on both microstructure and mechanical properties of this type of steel. The obtained results indicated that
refinement of the microstructure and consequently modification on mechanical properties was achieved. The grain size of
both proeutectiod ferrite and pearlite was decreased and the completely lamellar pearlite started in disappearance after the
first cycle. By increasing number of cycles the amount of lamellar pearlite decreased and on the other hand the amount of
cementite spheroids in the pearlite grains increased. Also, the inter-lamellar spacing of lamellar pearlite decreased from
0.75 to 0.28 μm by this type of heat treatment after 10 cycles. As a result of these microstructure modifications, both
hardness and ultimate tensile strength improved and in the same time the high ductility was retained.

Hydrogen-induced cracking (HIC) susceptibility in the heat-affected zone
(HAZ) was investigated and modeled using implant static tensile limit stress (σimp)
and maximum hardness of the HAZ (HV10MAX). C-Mn and high-strength low-alloy
(HSLA) steels were used as base metals with a carbon equivalent (CE) ranging from
0.38 to 0.48% and 0.52 to 0.69%, respectively. The shielded metal arc welding
(SMAW) and gas metal arc welding (GMAW) processes with CO2 shielding gas were
used. The diffusible hydrogen (H) content was varied taking the values between 2
and 40 mL/100 g. σimp and HV10MAX were the two measures used to evaluate the
weldment susceptibility to HIC. Using Pearson’s product-moment coefficient (Ppm)
and the developed analysis of HIC susceptibility, two simplified models were developed
using simple mechanistic models, linear and logarithmic, to simulate σimp and
HV10MAX. σimp was modeled as a function of HV10MAX and H, while HV10MAX was
modeled as a function of CE and t800/500. The two new simplified models were capable
of accurately simulating both σimp and HV10MAX. The newly developed
models form a simplified tool that can be used to assess HIC susceptibility in steel
weldments.

In this research, a comparative study has been carried out to investigate the effectiveness of
using different methods for hardfacing; namely surface treatment and welding. In surface
treatment method, quenching techniques was employed where specimen were heated and then
quenched directly in a proper medium. Heating temperature and quenching medium were
varied. 500, 700 927 oC were applied as heating temperatures and water and oil were applied
as quenching medium. Effects of each parameter on hardfaced surfaces were investigated.
Welding processes were varied and investigated through the application of SMAW and TIG
welding processes. Depositing techniques for hardfacing layers were also varied. Single,
parallel double and crossing double layers were employed for depositing techniques
comparison. Microhardness measurements and microstructure examinations for the hardfaced
groups were carried out. It was found that using welding in hardfacing can raise the hardness
of the surface to reach abt. 340% of its original raw material value. It is expected to have
better wear resistance for the hardfaced surfaces by welding. Surface treatment by quenching
in water after heating to 927 oC could raise the hardness values to abt. 200% of its original
raw material value. Economic study was made to calculate and determine the point at which
the cost of using special high strength steel and the cost of using soft steel with hardfacing
breakeven. This point was found to be located at material thickness of 13.5 mm; above this
value the application of hardfacing by SMAW is economic.

The operational life of a structure or a component is often limited by the initiation and subsequent
growth of cracks. The presence of cracks may result in a loss of performance or even in a total
failure of the structure therefore, it is recommended to -at least- delay the crack propagation if its
total prevention should be impossible. This delaying could be achieved by removing the highlystressed
area in front of the crack tip us ing a drilling- hole technique. In this study finite element
analysis has been carried out to investigate the stress and strain behavior at crack-tip region for
double edge notched tension specimens, DENT. The results of this finite element models have
been compared with the previous forms –proposed in the literatures- which describe the nature of
the plastic zone at the crack tip. Specimens with and without hole have been used with different
diameters and positions apart from the notch tip to determine the optimum condition for hole
existence. A set of experimental tests were carried out based on the present FE results. Good
agreement was found between both results of FE-analysis and experimental tests. This work
could conclude that the existence of a hole in front of a notch tip delayed the crack propagation
with different degrees depending on hole diameter and position. Optimum condition was
obtained when hole diameter equals plastic zone size, rp, and its center is located at a distance
equals to 2rp from the notch tip. The method developed in this work can serve as a quick remedy
in situations where unexpected crack is initiated. This will definitely minimize the losses until
further corrective steps are available.

The aim of this study is to evaluate the effects of food-simulating liquids on flexural strength and hardness of composites and polyacid-modified composite restorative materials. Four composites [Two hybrids, Spectrum TPH (STPH) and Tetric Ceram (TC) one packable, Tetric Ceram HB (TCHB); and one flowable, Tetric Flow (TF)] and one polyacid-modified composite [Compoglass F (CGF)] were used for this study. Specimens for flexural strength and microhardness testing were prepared. After light polymerization, the specimens were removed from their molds and conditioned for one week at 37°C as follows: (1) Air (control), (2) distilled water, (3) 0.02 M lactic acid, (4) Heptane, (5) 50% ethanol-water solution. After conditioning, flexural strength and microhrdness testing were carried out. Data were analyzed using ANOVA and post-hoc Tukey HSD tests at a significance level 0.05. The flexural strength of all restoratives was not affected after conditioning in heptane, which at the same time was significantly greater than that after conditioning in all other mediums. With the exception of Spectrum TPH, the flexural strength of all restoratives conditioned in ethanol-water solution was found significantly decreased compared to that conditioned in water and lactic acid. In all conditioning mediums used, Spectrum TPH had the significantly highest flexural strength and Compoglass F had the lowest. With the exception of Tetric Flow, no significant changes in surface hardness was noted with conditioning of all other restoratives in water, lactic acid, and heptane; however the surface hardness of all materials was significantly deteriorated after conditioning in ethanol-water solution. For comparing the materials after conditioning in various food-simulating liquids, Tetric Ceram HB was found to have the highest Vickers hardness number (VHN) and Tetric Flow had the lowest. Ethanol-water solution was found the only food-simulating liquid, which had a significant deterioration effect on flexural strength and hardness of all materials tested at the same time. Despite flexural strength values of composite materials were reduced after conditioning in water and lactic acid, it was still within the limits that can be accepted clinically.

The aim of this study is to clarify the effect of different concentrations of titanium dioxide nanoparticles (Nps) on the properties of two types of heat polymerized acrylic resin. The tested parameters were flexural strength, impact strength, and microhardness. The two types of acrylic resin used in this study were conventional unmodified (Implacryl, Vertex) and high impact heat polymerized acrylic resin (Vertex-Dental, Netherlands). Both types of acrylic resin were modified by using 1% and 5% TiO2 Nps powder. Specimen's dimensions were prepared according to the American Dental Association Specification No. 12. Three types of specimens were prepared: 1) flexural strength specimens 50 mm × 10 (±0.2) mm × 3 (±0.2) mm, 2) impact strength test specimens 60 mm × 6.0 mm × 4.0 mm, 3) microhardnesss specimens 25 mm × 10 mm × 3 (±0.2) mm. For each test 6 groups were prepared (each group containing 5 samples). Thirty specimens were prepared in each of the three tests, amounting to a total number of 90 specimens. Mechanical properties such as flexural strength (FS), impact strength and microhardness of the above mentioned specimens were determined using universal testing machine, Izod pendulum impact testing machine and Vickers microhardness tester, respectively. ISO Specification No. 1567 was followed in microhardness test. The data was collected and statistically analyzed. Flexural strength considerably decreased by increasing TiO2 concentration in both types of acrylic resin. Impact strength of the conventional acrylic resin modified by 1% of additives significantly increased. The microhardness is significantly increased by addition of 5% of TiO2 Nps. The Incorporation of TiO2 nanoparticles into acrylic resins can adversely affect its flexural strength. Meanwhile, the impact

In this research experimental study has been carried out to investigate various procedures for welding ductile cast iron (DCI) as similar and dissimilar welding. Preliminary experiments were conducted which showed failed welded specimens in many cases where shielded-metal arc welding (SMAW) process using AWS 6013 welding electrode was employed. In the light of these results, new and modified welding procedures were applied using both SMAW and tungsten inert-gas (TIG) welding processes in both similar and dissimilar joints. Dissimilar joints included joints with a combination between DCI and ST37 and DCI with stainless steel 304 (ST. ST. 304). Dissimilarity was not only in the welded steel type but also in the welded thickness. Variety of different thicknesses was joined together; namely 6, 7 and 8 mm plate thickness. Welded specimens were tested visually at the beginning. Then, examination of both microstructure and microhardness of welded joints were carried out. Tensile strength, macrohardness and impact value for selected welded specimens were determined and compared with microstructure and microhardness results. Successful similar and dissimilar welded joints proved the applicability of using the new welding techniques for DCI.

Hydrogen-induced cracking (HIC) susceptibility in the heat-affected zone (HAZ) was investigated and modeled using implant static tensile limit stress (σimp) and maximum hardness of the HAZ (HV10MAX). C-Mn and high-strength low-alloy (HSLA) steels were used as base metals with a carbon equivalent (CE) ranging from 0.38 to 0.48% and 0.52 to 0.69%, respectively. The shielded metal arc welding (SMAW) and gas metal arc welding (GMAW) processes with CO2 shielding gas were used. The diffusible hydrogen (H) content was varied taking the values between 2 and 40 mL/100 g. σimp and HV10MAX were the two measures used to evaluate the weldment susceptibility to HIC. Using Pearson’s product-moment coefficient (Ppm) and the developed analysis of HIC susceptibility, two simplified models were developed using simple mechanistic models, linear and logarithmic, to simulate σimp and HV10MAX. σimp was modeled as a function of HV10MAX and H, while HV10MAX was modeled as a function of CE and t800/500. The ...

Heat treatable alloy AA6082 is a medium strength alloy with have excellent formability from simple to complex profiles by extrusion and good corrosion resistance characteristics. It is one of the most widely used alloys and has considerable industrial interest. These materials can be heat treated to produce precipitation to various degrees. Mg and Si are the major solutes they increase the strength of the alloy by precipitation hardening. This article presents a survey for the main work done concerning welding and heat treatment carried out to this alloy. Metallurgical and mechanical properties of the AA6082 similar and dissimilar welds were reviewed.

High Cr white cast irons are an important class of wear resistant materials currently used in a variety of applications that requires high wear resistance and reasonable toughness. The outstanding performance of these alloys is due to the presence of large amounts of chromium carbides which exhibit high hardness. The size, type and morphology of these carbides control the wear resistance and toughness. The microstructural characteristics of these alloys, and consequently their wear resistance, can be extensively changed by varying the chemical composition or the solidification rate or by specific heat treatment after casting. Heat treatments for all high-Cr WCI alloys are essential to change their microstructure and therefore, to improve their wear resistance to suit the individual application requirements. Changing in chemical composition and heat treatment carried out to this alloy related to microstructural characteristics and mechanical properties of high Cr white cast iron allo...