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ROB LaPOINTE FASTENER SCIENCE: THE ROLE OF CARBON IN STEEL from page 14
Low Alloy Steel
Low alloy steel consists of low carbon, medium
carbon, and high carbon steels where carbon is the
main alloying element. Low carbon steel (0.04-0.3 %
C) has the lowest hardenability of low alloy steels. It
is characterized by its flexibility, machinability, and
low cost. SAE J429, grades 1 and 2 can be produced
from low carbon steel and do not need heat treatment
to achieve the tensile performance characteristics of
60,000 psi and 74,000 psi respectively.
FIGURE 3 SAE J429, GRADE 8 (MEDIUM CARBON) HEX CAP SCREW.
FIGURE 2 SAE J429, GRADE 2 (LOW CARBON) BOLTS. NO HEAD
MARKING REQUIRED.
Medium carbon steel (0.3-0.7 % C) is characterized
by a higher cost than low carbon steel and has good
hardenability through heat treatment involving a quench
and temper process. Medium carbon steels are often
used to produce SAE J429, grade 5 and 8 fasteners as
well as ISO 898-1, classes 8.8 and 10.9 bolts, screws,
and studs.
High carbon steel (0.7-1.7% C) has greater
hardenability than medium carbon steel for applications
requiring greater wear resistance and strength. This
added hardenability comes at the expense of flexibility
and cost. Applications suitable for high carbon steel
include cutting tools, knives, springs, and railway tracks.
High Alloy Steel
High-alloy steels have additional alloying elements
beyond carbon to improve the alloy’s workability,
hardenability ware resistance, and corrosion resistance.
Alloying elements include silicon (Si), titanium (Ti),
vanadium (V), chromium (Cr), manganese (Mn), Nickel
(Ni), copper (Cu), and molybdenum (Mo) among others.
Often simply called alloy steel, high alloy steels can
be used in producing SAE J429, grade 8, ASTM A574
socket head cap screws, and ISO 898-1, classes 8.8,
10.9 and 12.9 fasteners.
Stainless fasteners are produced from high alloy steel
with high chromium and nickel content. Specifically, it
is the high chromium content that gives stainless its
corrosion resistance. During the process of passivation,
iron is removed from the surface of the alloy by
interaction with an acid and chromium is left to form a
protecting layer of chromic oxide (Cr2O3) which shields
the material from oxidation of iron (red rust) deeper
under the surface. See Figure 4.
FIGURE 4 PASSIVATION PROCESS FOR STAINLESS STEEL, WHERE
CHROMIC OXIDE FORMS ON SURFACE AND PREVENTS IRON
OXIDES FROM FORMING BELOW.
CONTINUED ON PAGE 132