Jominy Test - Lab 6
MME 223- Dr. Hamilton
6 May 2015
Adam Cole
Matt Derickson
Abstract:
This lab analyzed the cooling curves of 3 different steel bars with varying carbon
content as well as an alloyed metal bar with similar carbon content to one of the other 3. A
combination of water quenching along with air quenching was used to get a gradual range
of cooling temperatures and times. The cooling curves gives ample information as to what
microstructure is most prevalent at 3 different places along the steel bar. From the cooling
curves we can see what type of structure is most likely to form. Normally with quenching
at faster temperatures, you would notice more martensite as well as bainite but the speed
at which this lab was conducted was too slow to have a majority of this microstructure. It
was shown by the plots that increasing carbon content ultimately increase the hardness of
the metal. The alloyed steel bar was originally harder than the similar carbon content bar
but when quenched slowly, the plain steel bar was harder than the alloyed steel.
Introduction:
The objective of this lab was to analyze the hardness and microstructure of four different
steel alloy bars that were austenitized and quenched with air and water. The four bars were
heated to a temperature that allowed the metals to reach the temperature needed to have been
solid austenite. They were individually cooled on one end by water while the other end was left
to cool by air. Hardnesses were measured (using the Rockwell Hardness test with the G scale)
along the length of the bar to see how the different cooling rates affected the microstructure of
the metal. The data that was collected was used to compare the different types of alloying bars
mechanical properties. The cooling rates helped show the makeup at given lengths along the
bar and they show the different properties that related to specific microstructures of the steel
alloy bars. Differing carbon content was compared to see how increasing the amount of carbon
would affect the overall hardness of the alloy and a comparison between similar carbon content
bars but one being alloyed metal rather than just regular steel. The information gathered helps
to better understand how different types of microstructures can affect hardness.
Results:
The formation of different microstructures becomes much more apparent once the data
is plotted along with similar steel bars. Referring to figure 1, one can see that the hardness near
the water-quenched end is always higher than the air quenched side. This would suggest that
the quicker you cool the steel bar, the harder the area becomes, whereas if you cooled the
metal slowly, the hardness will be noticeably less than a fast quenched steel. It can be shown
that when you cool at a much faster rate, the formation of martensite is greatly improved.
However, from the data collected in lab, it should be noted that the quench time was
approximately 10 minutes and was not nearly quick enough for noticeable amounts of
martensite to form. Rather, pearlite and small amounts of bainite are much more likely to be the
prominent microstructures of the steel bars.
As the carbon content of the steel bars were increased, the hardness of the bar
increased as well.
The interstitial carbon provided improved mechanical properties and
characteristics and was apparent in the graph comparing all the steel bars with differing carbon
content. The difference between the similar carbon content and alloyed metal is a bit more
complex and is represented by figure 2. It can be noted that the cooling curve of the nonalloyed metal appears to be decreasing linearly where the alloyed metal loses hardness rather
quickly if the metal isn’t cooled very rapidly. This would indicate that the formation of harder
microstructures must be done very quickly for alloyed metals that have more than just iron and
steel. The plain steel however retains harder microstructures than the alloyed steel at slower
cooling rates.
Conclusion:
From this experiment, we were able to see that steel with higher carbon content have
higher hardness. This is most likely because as more carbon are found, We also found that
differing alloys with similar carbon content vary as well. An alloy containing just carbon will have
higher hardness than the other steel alloy at points where quenching is slower.
Tables/Figures:
Figure 1
This graph shows the hardness at the water end, midpoint, and air quenched of 3 steel bars
varying in carbon content.
Figure 2
This graph represents the different hardness points of two bars of similar carbon content, one,
which is alloyed, and one that is not.
Table 1
Quench
Distance
Rockwell Hardness (G Scale)
1018
Hardness
1045
Tensil
Hardness
e
1060
Tensil
4340
Hardness
Tensile
Hardness
Tensile
e
water
1
60
75
101
98
125
86.5
middle
2
59
51
74
74
101
36
air
3
52
37
N/A
69.5
93
27
This table shows the hardness measurements of all 4 steel bars as well as some of the tensile
strengths (note: groups from 1018 and 4340 did not give tensile strengths).