Low Temperature Surface Carburization of Stainless Steels Page: 62 of 119

                                
4.1.5 Mechanical Testing

4.1.5.1 Tensile Tests
Tensile specimens (0.25-in.-round tensile bars) were fabricated from 316, 321, and 254 SMO SS.
Some were carburized using the standard LTCSS method, and others were not. Three specimens
from each steel sample in the as-received state and after carburization were pulled in tension.
The behaviors are summarized in Table 4.6, and an example showing the stress-strain behavior
of each sample is shown in Fig. 4.30. It is clear that the carburization process leads to a decrease
in the initial yielding of each steel. As a result, the initial linear behavior extends through higher
stresses/strains, and the resulting 0.2% offset yield stress is increased. The enhanced mechanical
properties (increased hardness, increased residual compressive stresses) of the carburized case
lead to a profound change in the low strain-stress/strain behavior. Several carburized specimens
were ground such that the entire carburized case was removed and finished in the same manner
as the specimens whose behavior is shown in Fig. 4.30. They were then pulled in tension, and
the stress-strain behavior compared with identical specimens that had their carburized cases
intact. As shown in Fig. 4.31, there is no difference in the stress-strain curves between the two
types of specimens. The delayed yielding of the carburized specimens must be due to the heat
treatment associated with carburization, and not the presence of the carburized case. All of the
specimens were carburized in the cold-worked state. Apparently, mobile dislocations present in
cold-worked samples were "pinned" during carburization (perhaps due to the low-carbon content
in as-received samples, leading to the higher 0.2% yield strength.
Table 4.6. The 0.2% Yield Strength (YS), Ultimate Tensile Strength (UTS), and %
Elongation upon Fracture for the Three Steel Alloys
Alloy                 0.2% YS              UTS                 %
(ksi)               (ksi)          Elongation
316 - Uncarburized             81.4 6.6           104.7 0.51          37.5 0
316 - LTCSS                    93.5 +2.1           113.2 +1.0        30.7 +0.9
321 - Uncarburized             58.8 +2.6          96.6 +0.34           50.0
321 - LTCSS                    66.3 +1.4          99.91 +0.5         47.5 +1.8
254SMO - Uncarburized          118.4 0.4          140.8 +1.0         25.5 +0.9
254SMO - LTCSS                 128.8 +3.0         144.5 +1.8         27.1 +0.9
Tensile tests were also performed on thin foil samples [0.002-in. (50- m) thick], made from
annealed 316 SS (Fig. 4.32). The yield stress is greatly improved. The ductility is decreased, but
this is a result of the tensile stresses that are generated in the outer portion of the foil, in contrast
to the compressive stresses that occur for thicker samples. Figure 4.33 shows the fracture
surface of a carburized thin foil sample tested in tension, showing ductile failure.

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