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

below:

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.

fr.

Mechanical Engineers’

Handbook: Materials and Mechanical Design,

Volume 1, Third Edition.

Edited by Myer Kutz

JASA SKETCHUP