Introduction
The following is a basic report with regards to three specific questions which will be answered, centered on material studies. The first question is based around work hardening, the changes in the atomic structure as a result thereof and the reason why some materials are more susceptible to work hardening than others. The second question is based on the process of embrittlement, as well as listing a few different ways in which embrittlement can occur. The third and last question is based on the significance of heat affected zones, with special emphasis on the effects with regards to steel.

Work hardening
Work hardening is a phenomenon found in metallic materials, where deformations in the metals have led to the metal itself becoming strengthened, or harder as such. The deformations which cause this effect are called plastic deformations, which means that the metal material was stressed beyond the point where elastic deformation takes place, thereby resulting in a permanent deformation in the crystalline structure of the metal material. These plastic deformations are caused by high heat exposure for a specific minimum length of time, causing the molecules within the crystalline structure to rearrange themselves. A few common physical processes which take place on metals and can cause this effect are as follows: hammering, bending, rolling, drawing, shearing, squeezing or collisions between metals for example. All of the above would result in some form of work hardening in metallic materials susceptible to it, due to the fact that large forces are applied during those processes, over relatively small areas of the material, causing high levels of heat in a localized area. Work hardening can have either a positive or a negative effect with respect to the material, which depends on the specific purpose of that material. Harder materials become more brittle, which means that if enough stress is applied, the material will shatter instead of conventional deformation, which can be a negative outcome. However, many materials are purposefully work hardened for applications which require harder materials, which then results in the hardening becoming a positive phenomenon.
The reasons why these deformations result in this phenomenon are due to the crystalline structure of the material, which is the term given to the unique arrangement of the atoms in a solid material in such a way as to create symmetry and order. When plastic deformation takes place, some regions within the crystals structure become displaced, otherwise known as slipping. These regions then serve as a barrier against further slippage, due to their displacement relative to the original crystal structure. This barrier then increases the hardness of the material, which is basically a measurement of how easily the crystalline structure can be manipulated or deformed.
It has become apparent over time that some materials are more susceptible to work hardening than others. It has been found that most carbon alloys, stainless steels and super alloys are highly susceptible to work hardening. The reason for this is believed to be due to the relatively small grain size of these materials as well as the relatively low shear modulus value of these materials, which is a measure of shear stress to shear strain, which is proportional to the strain hardening coefficient of a material.

Embrittlement
Embrittlement is the process whereby a material loses its ductility, which is a measure of the ability of a material to deform, and as a result the material becomes more brittle. This embrittlement often results in catastrophic cracks or failures in the materials if they are used after embrittlement has occurred. There are many different ways in which this embrittlement process can happen. For e.g. Hydrogen embrittlement, Sulphide stress cracking, Liquid metal embrittlement and Metal induced embrittlement. Embrittlement can be caused by many different factors. Decreasing or increasing temperatures, a change in the internal structure of a material such as the material grain size for example, corrosive environments, the presence of surface notches as well as metal embrittlement are all causes of embrittlement in materials. Each of these processes occurs in a different manner, but in the end the final result is the stated decreased ductility and increased brittleness.
The most common of the above mentioned processes of embrittlement is hydrogen embrittlement. What occurs on an atomic scale is that molecular hydrogen diffuses through the metal molecules, into deeper layers of the material due to the fact that molecular hydrogen molecules are extremely small, to later collect in groups of up to twelve atoms (Factors affecting hydrogen embrittlement) trapped in between complexes of metal atoms, and which then exert a force on the surrounding molecules and result in a weak spot in the metal material, thereby resulting in an area more likely to permanently deform with either cracks or fractures thus leading to the material failing. The reason why the presence of these external atoms in the crystalline structure easily results in failure of the material is due to the fact that during any process where stress or force is applied to the material, the molecules in the lattice around the hydrogen molecules would experience a different level of stress to the rest of the material, resulting in a weak point.
For hydrogen embrittlement to occur, molecular hydrogen needs to be present in the immediate environment around the material. This most commonly occurs in processes such as cathodic protection, pickling, electroplating and phosphating. Two other special cases also exist where hydrogen can be introduced. One is during arc welding, where hydrogen is released from any moisture involved in the process. The second case is galvanic corrosion, whereby chemical reactions between the metal material and some acids such as hydrogen Sulphide for example result in a release of hydrogen.

Heat affected zones during steel welding
During the process of welding, extremely high temperatures are made use of in order for the welding process to occur. These high temperatures have to be sufficiently high enough in order to allow for the work piece as well as the filler, otherwise known as the welding rod to be able to melt. The effect of this is that the areas adjacent to the welding spot are also subjected to this high heat, or at least a portion thereof. The heat in these areas is often sufficient enough to cause alterations in the material’s microstructure. The areas where these alterations take place are then known as heat affected zones (HAZ’s).
The alterations which may occur could for example be recrystallization or grain growth, both of which result in a portion of the metal’s crystal lattice which is different from the rest, resulting in a weak point against possible stress and force applications. This weak point therefore reduces the overall strength, hardness and toughness of the material. With regards to steel, the reason why HAZ’s are so significant is due to the fact that the temperature to which the work material is exposed is sufficiently high enough in order to result in the formation of austenite from the steel. Other carbon steels could also form, such as pearlite, proeutectoid and martensite. This is especially undesirable in the case of martensite due to the fact that it is extremely brittle.

Bibliography
Degarmo, E., Black, J. T., & Kosher, R. A. (2003). Materials and Processes in Manufacturing (9th ed). Wiley.
Embrittlement. (n.d.). Retrieved February 19, 2013, from http://www.mse.arizona.edu/undergraduates/files/embrittlement1.htm
Factors affecting hydrogen embrittlement. (n.d.). Retrieved February 19, 2013, from eHow: http://www.ehow.com/info_8598846_factors-affecting-hydrogen-embrittlement.html
Jewett, R. (1973). Hydrogen Environment Embrittlement of Metals. NASA.
Smith, W. F., & Hashemi, J. (2006). Foundations of Materials Science and Engineering (4th ed.). McGraw-Hill.
What is work hardening. (n.d.). Retrieved February 18, 2013, from Wise Geek: http://www.wisegeek.com/what-is-work-hardening.htm
Work (Strain) hardening - Strength (Mechanics) of materials. (n.d.). Retrieved February 18, 2013, from Engineers edge: http://www.engineersedge.com/material_science/work_strain_hardening.htm
Work hardening - A material kind of problem. (n.d.). Retrieved February 20, 2013, from The free library: http://www.thefreelibrary.com/Work+hardening--A+material+kind+of+problem.-a060808472