HAPTER C 14
1. Tempering softens metal.
2. Tempering is the process of reheating metal immediately after quenching for
t he purpose of increasing ductility or relieving stress. During the tempering
p rocess, the metal is heated to a temperature below i ts lower t ransformation
t emperature .This process is also called drawing or drawing back.
3. T he tempering also referred to the tendency of tempering to draw back the
hardness in steel.
4. T he purpose of tempering is to increase ductili ty and reduce internal stress.
5. D uctility is the property normally improved by tempering.
6. Tempering is not desirable for all applications .When hardness is important,
t empering may become a problem (when the internal stress not relief is
t empering not desirable).
7.
T he standard range in temperature use for tempering metal is between 3000
F and 12000 F i n a furnace.
8. A bout hour is a typical wait after a sample has been quenched before
beginning tempering.
9. A fter the material quenched, it is not t ransformed into other metallurgy
s tructures by tempering.
10. General quenching and tempering: when the material is quickly reheated
t o a tempering temperature between 5000 F a nd 10000 F, the steel reaches the
t empering temperature for several hours and is then quenched back to room
t emperature. The resulting structure is known as tempered mar tensite.
M ar tempering: when the steel is quenched, it is cooled rapidly at fi rst. However, it
is held at about 5000 F t o 6000 F for a short period of time .As a result, the effects of
t he quench are less drastic when the steel finally crosses the mar tensitic
t ransformation region.
11. T he special quenching method produces 100% bainite is austempering.
12. T he final structure of steel is produced by isothermal quenching and
t empering is consisting of bainite and tempered martensite.
13. I sothermal quenching is the special tempering process wherein a metal is
quenched to a temperature of about 5000 F t o 6000 F, held there for a few
seconds, quenched to martensite, and then tempered.
14. Austempering is the special tempering process wherein a metal is quenched
t o a temperature of about 5000 F to 6000 F, held at that temperature until
bainite is formed.
15. M ar tempering is of the four quenching and tempering process is considered
t o be the most drastic.
C HAPTER 15
1. The purpose of surface hardening is a heat-t reating process that creates a
t hin, hardened, wear-resistant layer on the outer surface of a material while
m aintaining a soft and ductile inner core.
2. Surface hardening commonly called case hardening , because a hard metal
case protects an unhardened core.
3. Name four different applications or components that require surface
hardening: ferri te, pearli te , cementite, or a combination of the structure.
4. T he differences between carburizing and nit r iding is the secondary heat
t reatment is not necessary in nit r iding.
5. T he two surfaces -hardening process use the principles of localized heating
a re: f lame hardening and induction hardening.
6. T he process of pack carburizing is the oldest and most basic surface
hardening process.
7.
Name two advantages gas carburizing has over pack carburizing are:
-
I t is a faster process
-
I t required less physical labor and handling.
8. T he carbon is deposited when steel parts are immersed in a carbon-r ich
l iquid. A molten bath salt is typically used.
9. Of all the surface- hardening process, which one is capable of producing the
hardest case?-nit r iding (over 70 on the RockWell C scale )
10. T he range of heating temperatures is typically used in nit r iding : 9000 F 10000 F.
11. Carbonit riding is a gaseous process that int roduces both carbon and nit rogen
t o steel parts.
12. W hich surface hardening process uses liquid to impregnate the surface of
s teel with carbon and nit rogen? Cyaniding (liquid carbonit riding ).
13. T wo surface hardening processes do not involve the addition of either carbon
or nit rogen to steel are: f lame hardening and induction hardening .
14. E xplain the advantages of f lame hardening over other surface hardening
p rocess :
Case depth: Flame hardening is a very rapid and efficient method for
p roducing case as deep as
Localized heating: selected areas of parts can be heated using the f lamehardening process. This method is very useful when only certain portions of a
part require hardening, suchas the jaws of a wrench. When a part is
quenched after f lame hardening, the chance of distortion is reduced since
only a small portion of the part has been heated. Tempering is still
r ecommended after quenching in order to reduce localized stress, but
d istortion of the entire part is minimized.
Cost: Flame hardening is a relatively inexpensive process. No expensive ovens
a re required, and no special chemicals or gases are used.
Small quantities of parts: are every suitable for f lame hardening. I t is usually
not economically practical to heat a large oven or salt both when only a few
parts need hardening.
Size of parts: Flame hardening is useful for large, bulky parts that cannot fit
i nto a furnace or tank. Large, heavy parts that cannot be t ransported
conveniently can be f lame hardened on location because the torch can be
t aken to them
Automation: The equipment required for f lame hardening can be as simple as
welding torch and a water hose. At the other extreme, highly mechanized,
automated systems are used to produce f lame hardening parts. Automated
systems are typically designed with combination tools that serve as both a
t orch and cooling source. After selected areas of the parts are hardened, a
spray of water quenches the part. This produces a fairly uniform case depth.
I n some systems, the parts may be immersed in a quenching medium, rather
t han sprayed. An automated system used to harden gear teeth.
15. The main disadvantage of cyaniding can be a very hazardous process.
Cyanide salted are poisonous, the resulting fumes can be fatal if they are
i nhaled. The areas surrounding the furnace must be well ventilated.
Proper care and safety must be practiced when handling material to be cyanided. If
l iquid from the salt bath makes contract with an open cut or wound, the results can
be very serious.
16. In induction hardening what is the source of electromagnetic induction?
In induction hardening the source of electromagnetic induction is the part to be
hardened is surrounded by a coil of wire, the coil acts like the primary winding of a
t ransformer. H igh frequency electrical current (3,000Hz- 1,000,000Hz) passes
t hrough the coil, producing a magnetic field around the parts. The electrical
r esistance of the part generates heat.
17. Induction hardening process is considered to be most accurate.