Hardenability Concept.

In thick products, e.g., large-diameter bars, thick plates, and heavy
forgings, the through-thickness properties are achieved through hardenability. Hardenability
is the ability to induce depth of hardness in a steel product. The hardness level is obtained
by controlling the amount of martensite in the microstructure. To increase the depth of
hardness, certain alloying elements are added to the steel for increased hardenability. Elements,
such as nickel, chromium, and molybdenum, shift the pearlite nose of the IT and CT
diagrams to the right (longer times). With the nose out of the way on cooling, martensite
can be formed over a wider range of cooling rates when compared with a steel without
alloying elements.
There is a fairly simple test to measure the hardenability of steel called the Jominy test.
A 25.4-mm-diameter and 102-mm-long bar is austenitized to 845 C for 1 h and then water
quenched at one end of the bar. The quenching takes place in a specially designed fixture
where the bar is suspended in the vertical position and water is directed against the machined
bottom end face of the bar. After quenching, parallel flats 0.38 mm deep are machined on
opposite sides of the bar. Hardness is measured at 1.6-mm intervals (–1–-in.) from the 16
quenched end. The hardness is plotted against depth from the quenched end to produce a
hardenability curve or band. A hardenability band for medium-carbon Society of Automotive
Engineers/American Iron and Steel Institute (SAE/AISI) 1045 steel is shown in Fig. 10a.
The two curves that form the band represent the maximum and minimum hardness values
from many Jominy tests. To illustrate the concept of hardenability, compare the hardenability
band for SAE/AISI 1045 steel to low-alloy SAE/AISI 4145 steel in Fig. 10b. These steels
are similar except that the low-alloy steel has chromium and molybdenum additions as shown

C Mn Si Cr Mo
0.42/0.51 0.50/1.00 0.15/0.35 — —
0.42/0.49 0.65/1.10 0.15/0.35 0.75/1.20 0.15/0.25
As can be seen from the hardenability bands, the higher manganese, chromium, and molybdenum
additions in the SAE/AISI 4145 steel produced a much greater depth of hardness
than the plain-carbon steel. For example, a hardness of HRC 45 (Rockwell C scale) was
achieved at a depth of only 3–6.5 mm in the SAE/AISI 1045 steel compared with a hardness
of HRC 45 at a depth of 21–50 mm in the SAE/AISI 4145 steel. This low-alloy steel has
many times the depth of hardness or hardenability of the plain-carbon steel. This means that
a hardness of HRC 45 can be achieved in the center of a 100-mm-diameter bar of SAE/
AISI 4145 steel compared to a 10-mm-diameter bar of SAE/AISI 1045 steel (both water
quenched). The depth of hardness is produced by forming martensite near the quenched end
of the bar with mixtures of martensite and bainite further in from the end and eventually
bainite at the maximum depth of hardness. Hardenability is important since hardness is
roughly proportional to tensile strength.

Mechanical Engineers’
Handbook: Materials and Mechanical Design,
Volume 1, Third Edition.
Edited by Myer Kutz


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