These materials utilize alloy materials including silicon, chromium, molybdenum, copper, and nickel at very low levels; and microalloying materials including columbium, vanadium, titanium, and zirconium in various combinations to produce a low-carbon steel with relatively high strength and good formability, weldability, and toughness. In effect, high-strength, low-alloy (HSLA) steels provide the best of both worlds, and come close to achieving this goal in many uses.
In practice, HSLA steels tend to be similar to mild steels in forming and drawing properties, but have less elongation tolerance, and are considerably more difficult to use in deep drawing operations. They also exhibit more springback than mild steels. HSLA steels are available as sheets and coils in a wide range of thickness and widths, all of which can be blanked, sheared, and slit.
(Manufacturing Engineering Handbook, H. Geng, p. 23-16)
Maraging Steel: (01:45 – 08.05.2011)
Maraging steels are a class of high-strength steels with very low carbon contents (0,030 wt % maximum) and additions of substitutional alloying elements that produce age hardening of iron-nickel martensites. The term maraging was derived from the combination of the words “martensite” and “age hardening”. Maraging steels have high hardenability and high strength combined with high toughness. The maraging steels have a nominal composition by weight of 18% Ni, 7 to 9% Co, 3 to 5% Mo, less than 1% Ti, and very low carbon contents. Carbon is considered an impurity and kept to as low a level as possible to minimize the formation of titanium carbide (TiC), which can adversely affect ductility and toughness. During air cooling from the annealing or hot working temperature, maraging steels transform to a relatively soft martensite (30 to 50 HRC), which can be easily machined or formed. They are then aged to high strength levels at 455 to 510 C (850 to 950 F) for times ranging from 3 to 9h.
The commercial maraging steels, 18Ni(200), 18Ni(250), 18Ni(300) and 18Ni(350) have nominal yield strengths after heat treatment of 1380, 1725, 2070 and 2415 MPa, respectively.
(Elements of metallurgy and engineering alloys ,Flake C. Campbell, p. 386)
Forehand Welding: (01:58 – 08.05.2011)
In the forehand welding method, the tip of the welding torch follows the welding rod in the direction in which the weld is being made (Figure). This method is characterized by wide semicircular movements of both the welding tip and the welding rod, which are manipulated so as to produce opposite oscillating movements. The flame is pointed in the direction of the weld but slightly downward so as to preheat the edges of the joint.
The major difficulty with the forehand welding method is encountered when welding thicker metals. In order to obtain adequate penetration and proper fusion of the Groove surfaces and to permit the movements of the tip and rod, a wide V-groove (90-degree included angle) must be created at the joint. This results is a large puddle, which can prove difficult to control, particularly in the overhead position.
(Welding Licensing Exam Study Guide, Rex Miller, Mark R. Miller, p. 94)
Frenkel Defect: (11:21 - 08.05.2011)
This type of defect is created when an ion leaves its normal lattice site and occupies an interstitial site. Since cations are usually smaller, it is more common to find the cations occupying interstitial sites.
A Frenkel defect in which a cation is missing from its regular site and occupying the interstitial site is shown in Figure.
Frenkel defect is common in ionic solids which have low co-ordination number and in which there is large difference in the size of cation and anion, for example, AgCl, AgBr, AgI and ZnS etc.
As no ion is missing from the crystal, as a whole, the density of the solid remains unchanged.
The presence of an ion in an interstitial position brings the similar charges closer. This results in an increase in the dielectric constant of the crystal.
(Essential Chemistry, V. P. Tyagi, p. 1.33)
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