Transient Nucleate Boiling and Its Use for Thermomechanical

Technologies Development

NIKOLAI I. KOBASKO

Intensive Technologies Ltd. Kyiv,

UKRAINE

Abstract: - In the paper high temperature and low temperature intensive thermomechanical treatment is

discussed. It is based on recently discovered new three physical principles that belong to the transient nucleate

thermo-mechanical treatment (HTTMT). It is used for low and middle-carbon alloy steels. Forged steel parts

are intensively quenched with a cooling interruption at the proper time to form surface compression residual

stresses and fine bainitic microstructure at the core that increases radically surface life of forgings. The second

method includes high-temperature and low temperature intensive thermo - mechanical treatment (LTTMT) of

high carbon alloy steels with delaying martensitic transformation to make low-temperature thermo - mechanical

treatment (LTTMT) possible. Then, after high temperature and low-temperature thermomechanical treatment,

the steel goes to immediate tempering to create highly strengthened fine bainitic microstructure throughout the

section of the steel part. A modified method of cooling time calculation, suitable for any size and form of steel

part, is widely discussed in this paper.

High and low temperature thermomechanical

treatment is known from literature and its practical

use in forging shops [1], [2] and [3]. As a rule,

thermomechanical treatment was performed in

slowly cooling oils to prevent crack formation.

Authors [4], used intensive cooling for direct

quenching of forging designated by them as DFIQ

process. As reported, [4], the intensive quenching of

forging immediately after hot forging operations

heat treatment costs. In addition, the DFIQ forgings

have very little surface scale, since the parts are not

reheated two times. It was noticed by the author, [2],

that the mechanical properties of AISI 1040 steel

after high-temperature accelerated

thermomechanical treatment are significantly

improved as compared with the conventional

hardening process (Table 1). The quenching after

forging was performed in cold water. One can

polymers of high concentration) to prevent crack

formation during forging and quenching.

Table 1. Mechanical properties of AISI 1040 steel for heavy rolling with 19 mm diameter in the case of high–

Low-temperature thermomechanical treatment is

performed in hot oils to delay the transformation of

cooling is intensive and is always suitable for plain

carbon steels and alloy steels of any chemical

composition. The technology is based on new

physical principles published in [7], [8] which sound

as:

● During quenching from high temperature in cold

fluid any film boiling can be absent completely if

initial heat flux density

.

successfully low-temperature thermomechanical

treatment.

If the initial heat flux density

boiling is completely absent which decreases

distortion of hardened steel parts, [7], [8]. Note,

shock boiling is present also at the beginning and at

the end of film boiling oscillating with high

If

accurate experimental data of author, [9], presented

in Table 2. The initial temperature of the transient

nucleate boiling process and the initial temperature

of convection are evaluated using equations (1) and

(2).

(1)

(2)

where

is critical pressure in Pa;

coefficient at nucleate boiling (

tension (

is gravity acceleration factor

density (

is heat flux density (W/m2);

is heat of vapor formation (J/kg);

is steam

bubble growth rate (m/s);

is initial temperature;

is kinematic viscosity (m2/s);

diffusivity of liquid (m2/s).

when initial

HTC is rather small and for still water and water salt

solutions reaches 400 - 1200 W/m2K. Convective

HTC on average is 200 times smaller as compared

with the nucleate boiling process. It means that

number during the transient nucleate boiling process.

To prove the theoretical existence of SRTP let’s

consider a well-known universal correlation (10),

[14], [15], [16]:

Here

average volume temperature,

is bath temperature,

Assume that the surface temperature of steel

part at the beginning of cooling is below the

saturation temperature TS and is in the convection

area. Taking into account Eq. (10), one can assume

that

. It means that surface temperature must

increase immediately if it drops below Ts and occurs

in the convection area. Assume now that the

by itself

Fig. 6: Cooling curves vs time during quenching

cylindrical sample 50 mm dia 200 mm long in cold

water solution (14%) of NaCl at room temperature.

Lines are numerical calculations; dots are

(Figure 7). Real Knnb and effective Knconv

Kondrat’ev numbers versus time. when cooling the

cylindrical sample 25 mm dia 120 mm height in

cold water salt solution of optimal concentration.

are within 0.2 – 1 for effective data and 0.8 - 1 for

real Kondrat’ev numbers, [8]. Their average values

are:

and

show that the transient nucleate boiling process

provides intensive quenching because Knreal >

Fig. 7: Real Knnb and effective Knconv Kondrat’ev

numbers versus time when cooling cylindrical

sample 50 mm dia 75 mm height in cold water

under pressure 0.7 MPa

The listed above technologies are possible if

To explore the transient nucleate boiling process

for new technologies development, convective HTC

should be reduced as much as possible, it increases

the duration of transient nucleate boiling.

The generalized correlation for optimizing the

salts. The optimization of aqueous salt solutions is

achieved by controlling the ionic charge that is

present at the interface of the quenchant and the

thoroughly while intensive HTTMT and LTTMT

are not investigated enough. The current paper asks

scientists put together their efforts to investigate

properly the intensive HTTMT and LTTMT

technologies.

As known, for all electrolytes there are optimal

Fig. 10: Maximal critical heat flux densities qcr1

versus concentration for NaCl and LiCl [7]: 1 is

NaCl; 2 is LiCl

Some water salt solutions are used in the heat

treating industry as quenchants to eliminate film

boiling processes that provide uniform cooling and

decrease distortion of steel parts after quenching.

Any water salt solution provides the maximum

value of the first critical heat flux density if the

pressurized fluid flow. Critical heat flux density in

this case can be increased by short-lasting external

negative electrical force directed to semi – the axle.

Currently, the technology is used for intensive

It can be used also for performing HTTMT and

polymer water solution of inverse solubility

6.4 Resonance Effect to Eliminate Local and

To eliminate film boiling processes one can use

special emitters that produce waves with frequencies

equal to the frequencies of film boiling processes.

The arrangement of emitters in the quench tank are

shown in Figure 13, [8].

tank with located in it emitters, where 1 is quench

tank; 2 is water salt solution of optimal

6.5 Effect of Intensive Quenching for

The effect of intensive quenching on surface

compression residual stress formation was for the

first time discussed by authors, [7]. Further accurate