When steel ingots are worked down into usable sizes and shapes such as billets and forging blanks, some inherent discontinuities may appear. But the rolling and forging processes may themselves introduce discontinuities which in many cases constitute defects. Primary processes are those which work the metal down, by either heat or cold deformation, into useful forms such as bars, rod and wire, and forged shapes. Casting is another process usually included in this group since, though it starts with molten metal, it results in a semi-finished product. Welding is also included for similar reasons.
Seams
Seams in rolled bars or drawn wire are usually highly objectionable and often downgrade the product and make it unusable for first quality purposes. Severe seams may originate from ingot cracks, but by proper cleaning up (conditioning) of the billets by scarfing, grinding or chipping these' can be eliminated before final rolling. Conditioning is usually aided by the use of magnetic particle testing to indicate the length and severity of the seams. Sec Chapter 19. If properly conditioned at the billet stage, seams from this source need not appear in the final rolled product. But seams can be introduced by the rolling or drawing processes themselves. Laps can occur in the rolling of the ingot into billets as the result of over-filling of the rolls. This produces projecting fins, which on subsequent passes are rolled into the surface of the billet or bar. When severe, the billet often cannot be salvaged and is downgraded. Similarly, even when billets have been conditioned and are free of seams, are rolled into bars or rods, laps result from over-filled rolls can occur, producing long and often very deep seams in the finished product. In similar fashion, under-fills in the rolling process may, on subsequent passes, be squeezed to from a seam which often runs the full length of the bar at an acute angle. Seams derived from the folds produced by an under-filled pass are likely to be nearly normal to the surface of the bar. Seams or die marks may also be introduced in the drawing process due to defective dies. Such seams may or may not make the product defective. For some purposes, such as springs or bards for heavy upsetting, the most minute surface imperfections or discontinuities are cause for rejection. For others, when for example, machine operations are expected to remove the outer layers of metal, seams which are not too deep will be machined away.
Laminations
Laminations in rolled plate or strips are formed when blowholes or internal fissures are not welded tight during rolling but are enlarged and flattened into areas of horizontal discontinuities. Laminations may be detected by magnetic particle testing on the cut edges of plate but do not give indications on plate or strip surfaces, since these discontinuities are interal and lie in a plane parralal to the surface. Ultrasonic inspection techniques can be used to locate and define them.
Cupping
This is a condition created when, in drawing or extruding a bar or shape, the interior of the metal docs not flow as rapidly as the surface. Segregation in the center of the bar usually contributes to the occurrence. The result is a series of internal ruptures which are severe defects whenever they occur. They may be indicated with magnetic particles, but only if the ruptures are large and approach the surface of the bar. The cupping problem can be minimized by changing die angles.
Cooling Cracks
When alloy and tool steel bars are rolled and subsequently run out onto a bed or table for cooling, stresses may be set up due to uneven cooling which can be severe enough to crack the bars. Such cracks are generally longitudinal, but not necessarily straight. They may be quite long and usually vary in depth along their length. Magnetic particle indication varies in intensity, being heavier at points where the crack is deepest.
Flakes
Flakes are internal ruptures that may occur in steel as the result of too rapid cooling. It is believed that the release of dissolved hydrogen gas during the cooling process causes these ruptures. and that controlled slow cooling after forging or otherwise hot working the metal will reduce their occurrence. Flaking usually occurs in fairly heavy sections and certain alloys are more susceptible! than others. Since these ruptures are deep in the metal-usually half way and more from the surface, they will not be shown by magnetic particle testing on the original surface of the part.
Forging Bursts
When steel is worked at improper temperatures it is subject to cracking or rupturing. Too rapid or too severe a reduction of temperature in a section can also cause bursts or cracks. Such ruptures may be internal bursts, or they may be cracks on the surface. When on the surface, they are readily found by magnetic particle testing. If within the interior, they are usually not shown except when they have been exposed by machining.
Forging Laps
As the name implies, forging laps or folds are formed when, in the forging operation, improper handling of the blank in the die causes the metal to flow so as to form a lap which is later squeezed tight. Since it is on the surface and is oxidized, this lap does not weld shut. This type of discontinuity is sometimes difficult to locate, because it may be open at the surface and fairly shallow, and often may lie at only a very slight angle to the surface. Jn some unusual cases it also may be solidly filled with magnetic oxides.
Buring
Overheating of forgings, to the point of incipient fusion, results in a condition which renders the forging unusable in most cases and is referred to as burning. Actual oxidation is, however, not the real source of the damage, but rather the partial liquidation due to heat, of material at the grain boundaries. Burning is a serious defect but is not generally shown by magnetic particle testing.
Flash Line Tears
Cracks or tears along the flash line of forgings are usually caused by improper trimming of the flash. lf shallow, they may "clean up" if machined, and do not make the part defective; or they may be too deep to clean up and in such cases the forging cannot be salvaged. Such cracks or tears can easily be found by magnetic particles.
Castings
Steel and iron castings are subject to a number of defects which magnetic particle testing can easily detect. Surface discontinuities are formed in castings due to stresses resulting from cooling and are often associated with changes in the cross section of the part. These may be hot tears or there may be shrinkage cracks which occur as the metal cools down. Sand from the mold, trapped by the hot metal, may form sand inclusions on or near the surface of castings. Gray iron castings may be quite brittle and are often cracked - usually at thin sections-during the "shake-out" or by rough handling during sorting.
Weldments
A number of kinds of discontinuities may be formed during welding of both thin and heavy sections. Some are at the surface am! some are in the interior of the metal. Some of the defects peculiar to weldments are lack of penetration, lack of fusion, undercutting, cracks in the weld metal, crater cracks, cracks in the heat affected zone, etc.
Want to learn more about discontinuities and defects? Read our detailed blog about the difference between the two.
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