June_AMP_Digital

ADVANCED MATERIALS & PROCESSES •

JUNE 2014

The size of a batch immersion quench

tank depends on the dimensions of the

workload, as well as the allowable tem-

perature rise. The temperature rise per-

mitted is dependent on whether the

quenchant is oil, water, or polymer.

In a batch operation, care should be

taken to ensure that a sufficient amount

of quenchant covers the top of the work-

load. The physical dimensions of the

tank should be large enough to ensure

full immersion of the quench load and

fixtures, and, at the same time, should

allow enough space for agitators and ma-

nipulators. Depending on the size of the

workload, it is generally appropriate to

have at least 150–300 mm (6–12 in.) of

fluid over the top of the workload, and

preferably more.

When using hot quenching oils, it is nec-

essary to make an allowance for thermal

expansion of the oil, either by making

provision for an overflow system, or by

manual adjustment of the fluid level.

Tank capacity

A quench tank must contain sufficient

fluid to quench the load without an ex-

cessive rise in temperature of the

quenching fluid. In an uncooled tank,

the quantity of quenchant required can

be calculated from the basic equation:

Where

is the mass of metal,

the specific heat of the metal, D

the decrease in temperature of the

metal being quenched,

is the mass

of quenchaant,

is the specific heat

of the quenchant, and D

is the in-

crease in temperature of the quen-

chant. Typical values for specific heat

at 20°C (70°F) are:

Steel — 0.17 cal/g/C (0.17 Btu/lb/F)

Aluminum — 0.23 cal/g/C

(0.23 Btu/lb/F)

Quench Oil — 0.50 cal/g/C

(0.50 Btu/lb/F)

Polymer Quenchant — 0.95 cal/g/C

(0.95 Btu/lb/F)

Water — 1.0 cal/g/C (1.0 Btu/lb/F)

A general guideline for steel quenching

(for a single quench) is that 10 liters of

oil is required for each kilogram of total

charge weight (1 gal/lb). This rule of

thumb results in about a 40°C (70°F)

temperature rise under nominal condi-

tions, which is recommended to prevent

the oil from reaching the flashpoint of

the fluid. It is recommended that the

maximum temperature during quench-

ing using oils always should be at least

55°C (100 °F) below the flash tempera-

ture, which mitigates the potential for a

fire if a hung-up load occurs. This is il-

lustrated in the following example:

Flash temperature of the oil:

175°C (350°F)

Recommended temperature

cushion: 55°C (100°F)

Temperature rise during quenching:

40°C (70°F)

Maximum recommended operating

temperature: 80°C (180°F)

However, with successive quenches,

some form of cooling is necessary to pre-

vent the oil from overheating. The heat

exchanger should be sized to recover the

heat produced by the quenched load

within one heat treating cycle.

For example, in quenching 2270 kg (5000

lb) of steel from 870°C (1600°F), a heat

treater wants to remove the parts from

the oil at 65°C (150°F). The “cold

quench” oil used has a flash temperature

of 175°C (350°F) and operates at a tem-

perature of 60°C (140°F). Based on these

process parameters, the maximum peak

temperature of the oil (considering the

flash point of 175°C), would be 120°C

(250°F). What should the minimum size

of the quench tank be? Using the equa-

tion given above:

therefore

= 5000 lb (0.17 Btu/lb/F)

(1600 - 150F)/0.50 Btu/lb/F (110F)

= 22,409 lb (10,185 kg) oil

At a weight of 6.8 lb/gal (0.8 kg/liter), ap-

proximately 3300 gal (12,500 liters) are

required for these conditions. However,

this temperature rise is a bit excessive

and could lead to premature oxidation of

the oil. Alternatively, rearranging the

equations using a fixed size quench tank

allows solving for the temperature rise

during quenching.

Water and polymer quenchants have a

different limitation on temperature. This

is not related to safety, as with quench

oils, but effective cooling for water, and

maximum operating temperatures for

polymer quenching. For water, the max-

imum temperature is 100°C (210°F).

However, this limit is rarely used as the

cost of make-up and cooling becomes

excessive. For polymer quenchants and

water used in quenching aluminum,

aerospace standards specify a maximum

SIZING QUENCH TANKS FOR

BATCH IMMERSION QUENCHING

HTPRO

*Member of ASM International and Heat Treating Society.

A QUENCH TANK MUST CONTAIN SUFFICIENT FLUID TO QUENCH THE LOAD

WITHOUT AN EXCESSIVE RISE IN TEMPERATURE OF THE QUENCHING FLUID.

D. Scott MacKenzie,* FASM,

Houghton International Inc., Valley Forge, Pa.

Hot charge ready to be quenched