2-276. Shearing and Blanking. Shearing and Blanking of corrosion resisting steels as with other fabrication processes
requires more power in comparison to shearing carbon steel and most other metals. Shears and other equipment rated
for carbon steel should not be used above 50 70% of rated capacity when cutting stainless.
2-277. Hot Forming. Hot forming is used to form shapes in stainless that cannot be accomplished by cold forming and
for forging parts economically. In using heat for forming, it is important that temperature be closely controlled. Also,
finished parts should be relieved of residual stress and carbide precipitation which affects corrosion resistance. In either
case, this is accomplished by fully annealing.
Difference in temper of raw material will result in variation of preheating, especially with the
air hardening grades. The air hardening grades in tempers other than annealed may crack
from thermal shock upon loading into a hot furnace.
2-278. Hot forming by methods other than forging is accomplished at somewhat lower temperatures. The unstabilized
chromium-nickel grades may be formed at temperatures up to 800°F and the extra low carbon grades up to 1000°F. The
use of temperatures higher than those cited above should be avoided to prevent subjection of material to the carbide
precipitation heat zone.
2-279. The straight chromium (type 400 series) are more responsive to hot forming than the chromium nickel grades.
The reaction of these metals to hot forming is similar to carbon steels. Upon heating to 800°-900°F, their tensile strength
is lowered considerably and at the same time ductility begins to increase.
2-280. Forming of the air hardening grades type 403, 410 is accomplished in two temperature ranges as follows:
Low temperature forming up to 1400°F. The advantage of forming at this temperature is that parts can be stress
relieved at 1350°- 1450°F to restore strength uniformity, and scaling is held at a minimum.
High temperature forming at 1525° -1575° F. Forming at this temperature is somewhat easier because strength
is low and ductility is higher. Upon completion of forming at this temperature, parts shall be fully annealed under
controlled conditions by heating to 1550°F and holding, slowly cooling to 1100°F (at approximately 50°F per hour) and
then cooling in air.
Grades 4, and 410 are not subject to loss of corrosion resistance due to the forming of intergranular carbides at
2-281. When it is required that the non-hardening grades 430, 442, and 446 be hot formed, the recommended
temperature for forming is 1400°-1500°F. This temperature is recommended in view of the following:
Heating these grades above 1600°F promotes grain growth which can only be corrected by cold working.
For types 442 and 446, the 1400° 1500°F temperature is below the scaling limit and very close to being below
the scaling limit for type 430.
2-282. STEEL SURFACE FINISNES.
2-283. Metal plating is a process where an item is coated with one or more thin layers of some other metal. This is the
type of finishes generally used on ferrous parts, other than organic finishes. It is usually specified when there is a need
for surface characteristics that the basic metal does not possess. The most commonly used types of plating are: (1)
Cadmium plate; (2) zinc plate; (3) nickel plate; (4) chromium plate; (5) copper plate; (6) tin plate; and (7) phosphate
coatings. The thickness of the plated coating is important since its protective value is primarily dependent on its
thickness. The type of plated coatings is generally dependent on the characteristics desired. For protection against
corrosion when appearance is unimportant, either cadmium or zinc coatings is usually used. For appearance, nickel,
chromium, and silver plating are the most commonly used. For hardness, wear resistance, and build up of worn parts,
nickel and chromium plating are used. Effectiveness of most other metallic coatings depends on their ability to provide
envelope or anodic protection. Porous coatings of the more noble metals such as silver, copper, platinum and gold, tend
to accelerate the corrosion of steel. For proceeding instructions, refer to T.O. 42C2-1-7. The following galvanic series
table and sHmsl,it1ar metal definition in accordance with M833586 are for use as a guide in the selection of the most
suitable plating for parts subject to uses where galvanic corrosion would be a prime factor.
2-284. DEFINITION OF DISSIMILIAR METALS. Dissimilar metals and alloys, for the purpose of aircraft and aircraft
parts construction are separated into four groups in accordance with MB33586. Metals classified in the same group are
considered similar to one another and materials classified in different