Machining removes any decarburised layer formed in the normalising treatment. The machined bar is put inside a closed box, half-filled with cast iron chips to prevent decarburisation, and then austenitised for 30 minutes at about 22-23°C above Ac3.
Meanwhile, the special water-quenching device is kept ready. In this device, the diameter of the pipe opening is 1/2 inch. Specimen holder is such that the bottom of the specimen remains at 1/2 inch above the nozzle. The constant pressure of the water is such that the height of the impinged jet is 2 inch.
The water temperature is 75 ± 5°F. The Jominy-bar after austenitising is transferred to the quenching-device in less than 5 seconds, and almost simultaneously the stream of water is then directed against the bottom face of the specimen for not less than 10 minutes.
The resulting curve is called ‘Jominy hardenability curve’ and the distance from the quenched-end is known as Jominy distance-J. (1/16″ = one Jominy distance = J1). Fig. 4.18 illustrates typical Jominy curves of some low alloy steels.
The inflection point (Fig. 4.18 for SAE 1040 steel) on this hardness versus distance curve usually corresponds to 50% martensite and is used as a measure of hardenability of the steels, or, knowing the carbon content, and thus, the hardness of 50% martensitic structure from Fig. 4.1, and then drawing a horizontal line at this hardness (HRC 43) in Fig. 4.18. The x-axis of each intersection point gives critical Jominy distance for that steel.
Important Aspect of Jominy End Quench Test
Jominy specimen. The basic postulate is that, it is the cooling rate that determines the amount of martensite, i.e., microstructure and thus, the hardness that develops al a given point in a steel specimen.
Therefore, if cooling rates as a function of position in parts of different diameters are known, it is possible to use Jominy curves to plot hardness profiles in the parts, i.e., two points in two pieces of same steel if cool at the same rate, the microstructure, the resulting hardness will be the same even in different quenching mediums.
Such correlations of cooling rate (by section-quench method) as a function of position in various sizes of bars and plates quenched in various media are available. The relationships between the 50% martensite position and the 3/4 radius, 1/2 radius, and the centre positions of oil-quenched bar (from 845°C
Calculation of Jominy Curve from Composition in Steels:-
As computers can handle complex equations easily, these are now being used to solve problems of computing hardenability. Using multiple regression analysis and proceeding from the chemical composition. Just has obtained equation to predict hardness as a function of Jominy distance to be calculated directly. It is observed that all alloying elements have an increasing effect up to a Jominy distance of about 10 mm, and then their effect remains practically constant.
Carbon starts off at a Jominy distance zero with a factor of 60, but all other alloying elements have zero factor at zero distance, i.e., Hardness at Jominy distance zero is solely governed by the carbon content. The factors vary negligibly for Jominy distances exceeding 7 mm.
This gives hardness in Rockwell C at J distances in range 4/16 to 40/16 inch from the quenched end. Elements are in wt%. E is the distance from the quench end in sixteenths of an inch and K is the ASTM grain size number. For less than 4/16 inch, C has the effect on hardness, and thus, following equation holds for end face of the Jominy specimen.
The above two equations can be evaluated by calculators to obtain the hardenability curve. Thus, computers can be used to generate this curve. The curves are reasonably satisfactory.
Breens equation for boron steels- hardness at Jominy distances is:
J1 = 39.5 + 37.5 C
J2 = 38.6 + 37.9 C
J3 = 38.1 + 37.8 C
J4 = 36.3 + 41.1 C
J5 = 23.6 + 44.6 C + 8Mn + 10.2 Cr
J6 = 5.3 + 58.0 C + 16.2 Mn + 30.4 Cr
J7 = -27.6 + 65.5 C + 35.1 Mn + 66.0 Cr
J8 = -40.7 + 54.4 C + 42.0 Mn + 93.6 Cr
J10 = – 37.5 + 39.2 C + 37.3 Mn + 73.9 Cr
J12 = -31.2 + 37.5 C+ 29.9 Mn + 54.8 Cr
The equations are valid in composition range:
C = 0.25 – 0.35%: Mn = 1.2 – 1.38%; 5/ = 0.23 to 0.35%
M = 0.02 to 0.04%; Cr = 0.02 – 0.12%; Mo = 0.01 to 0.02%
and B = .0005 minimum.
Hardenability Bands:-
A steel producer cannot commercially produce steel of exact given composition, and thus a composition range is specified while ordering to the producer. He produces a steel of one exact composition falling within the range of specification. This composition range for a given steel gives rise to two curves—the upper curve represents the hardness obtained with the upper limit of elements in composition for the steel, while the lower curve is that for the composition at the lower limit.
For Example:
1. 80% martensite at the centre of largest section is normally needed for highly stressed parts, whereas for less highly stressed parts, 50% martensite is sufficient.
2. 80% martensite at the 3/4 radius position is needed for most automotive parts..
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