Correct Answer - Option 3 : uneven expansion due to carbon deposits in the part
Heat treatment is the process of heating and cooling metal in its solid-state in order to obtain the desired changes in its physical properties.
- Heat treatment or the amount of heat treatment an alloy can undergo essentially depends upon the alloying elements.
- Once the metal is heated to a certain temperature then it is the cooling rate that decides the properties obtained by the heat treatment, for example in the annealing process the steel it is allowed to cool in the furnace itself, the austenite changes into coarse pearlite, and in the normalizing process the steel is allowed to cool in the open air then the austenite changes into fine pearlite.
- Most of the problems in heat-treated parts are attributed to faulty heat-treatment practices (such as overheating and burning, and nonuniform heating and quenching), deficiency in the grade of steel used, part defect, improper grinding, and/or poor part design.
Anything that produces excessive quenching stress is the basic cause of cracking. Quench cracking is mostly intergranular, and its formation may be related to some of the same factors that cause an intergranular fracture in overheated and burned steels. The main reasons for cracking in heat treatment are: part design, steel grades, part defects, heat-treating practice, and tempering practice.
Distortion in Heat treatment:
Distortion is a general term that involves all irreversible dimensional changes produced during heat-treatment operations.
This can be classified into two categories:
Size distortion, which is the net change in the specific volume between the parent and transformation product produced by phase transformation without a change in geometrical form.
- Shape distortion or warpage, which is a change in geometrical form or shape and is revealed by changes of curvature or curving, bending, twisting, and/or nonsymmetrical dimensional change without any volume change Usually, both types of distortion occur during a heat-treatment cycle.
- The carbon content of steel should not exceed the required level; otherwise, the risk of distortion & cracking will increase because of uneven expansion due to carbon deposits in the part.
- Because of the segregation of carbon and alloying elements, some steels are more prone than others to quench cracking.
- Steel Grades. Sometimes this can be checked by means of a spark test, whereas at other times a chemical analysis must be made.
Features such as sharp corners, the number, location, and size of holes, deep keyways, splines, and abrupt changes in section thickness within a part (that is, badly unbalanced section) enhance the crack formation because while the one (thin) area is cooling quickly in the quenchant, the other (thick) area immediately adjacent to it is cooling very slowly. One solution to this problem is to change the material so that a less drastic quenchant (for example, oil) can be employed. An alternate solution is to prequench, that is, to cool it prior to the rest of the part. This will produce an interior of the hole or keyway that is residually stressed in compression, which is always desirable for better fatigue properties. The third solution is a design change, and the fourth is to use a milder quenchant.
Surface defect or weakness in the material may also cause cracking, for example, deep surface seams or nonmetallic stringers in both hot-rolled and cold-finished bars. Other defects are inclusions, stamp marks, and so forth. For large-seam depths, it is advisable to use turned bars or even magnetic particle inspection. The forging defects in small forgings, such as seams, laps, flash line, or shearing crack, as well as in heavy forgings, such as hydrogen flakes and internal ruptures, aggravate cracking.Similarly, some casting defects, for example, in water-cooled castings, promote cracking.