Study of Jominy Hardenability Test and its Industrial use.
Objective
To study hardness as a
function of quench rate and investigate the hardenability of steels.
Introduction The
hardenability of a steel is defined as that property which determines the depth
and distribution of hardness induced by quenching from the austenitic
condition. The dependence of hardness upon quenching rate can be understood from
the time-temperature-transformation characteristics of steel, and, for a
particular steel, can be estimated from the T-T-T diagram.
A part may be hardened by
quenching into water, oil, or other suitable medium. The surface of the part is
cooled rapidly, resulting in high hardness, whereas the interior cools more
slowly and is not hardened. Because of the nature of the T-T-T diagram, the
hardness does not vary linearly from the outside to the center. Hardenability
refers to capacity of hardening (depth) rather than to maximum attainable
hardness.
The hardenability of a
steel depends on
(1) the composition of the
steel,
(2) the austenitic grain
size, and
(3) the structure of the
steel before quenching.
In general, hardenability
increases with carbon content and with alloy content. The most important factor
influencing the maximum hardness that can be obtained is mass of the metal
being quenched. In a small section, the heat is extracted quickly, thus
exceeding the critical cooling rate of the specific steel and this part would
thus be completely martensitic. The critical cooling rate is that rate of
cooling which must be exceeded to prevent formation of non-martensite products.
As section size increases, it becomes increasingly difficult to extract the
heat fast enough to exceed the critical cooling rate and thus avoid formation
of non-martensitic products. Hardenability of all steels is directly related to
critical cooling rates.
After the sample has been
austenitized, it is removed from the furnace and placed directly into the
quenching apparatus. A jet of water is quickly splashed at one end of the
specimen. After the entire sample has cooled to room temperature, the scale
oxidation is removed; two opposite and flat parallel surfaces are ground along
the length of the bar. Rockwell C hardness measurements are then made every 2
mm and these readings are recorded. Results Plot a hardenability curve of
Rockwell hardness vs. distance from the quenched end. (Fig. 3)
Low alloy steels have large
range of application in oil country tubular goods (OCTG). The main reasons for
those applications are excellent hardenability, high strength, a good toughness
and high resistance to sulfide stress corrosion cracking (SSCC) as the form of
hydrogen embrittlement (HE). The hardenability of steel can be defined in
several ways. It is the ability steel to acquire hardness being austenitized
and quenched. This definition emphasizes hardness. The source of hardening is
the formation and presence martensite structure. Thus, high hardness is related
to martensite formation. If the hardenability is low, fast cooling is needed to
achieve a large amount of martensite. However, fast cooling causes the
development residual stress, distortion and even cracking. Excess hardenability
usually represents excess cost. Thus, it is expedient to optimise the least
expensive and most efficient alloy system. A numerous of articles are described
on a numerical simulation of the Jominy end-quench test on simulate phase
transformation in the steel. Also, today the neural networks are using as a
tool for hardenability modelling. To the evaluation of hardenability is the use
the end-quench test which is referred to as the Jominy test. The test is widely
used to determine hardenabilities in the range round from 10 to 60 mm.
Discussion and Questions
1. Evaluate the
hardenability of the steel used in this experiment using the plotted
hardenability curve.
2. Predict the
microstructure of the steel all along the bar in correlation with your hardness
measurements. What is the ideal critical diameter and can it be determined with
a Jominy test.
3. How is the role of
carbon and various alloy elements on the hardenability of steels (Give examples
of different hardenability curves).
