UDC 621.785.533

USE OF DOUBLE THERMOCHEMICAL TREATMENT

IN THE PRODUCTION OF TOOLS FROM STRUCTURAL STEELS

L. P. Karpov1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 7 – 8, January, 2003.

It is shown that double thermochemical treatment (nitrocarburizing and nitriding) with subsequent heat treat-

ment can be used for the production of cutting tools from alloy structural steels instead of tool steels. A process

for treating milling cutters from steel 38Kh2MYuA and blades from steel 0KhN3MA is suggested.

INTRODUCTION

The use of alloy tool steels for the production of cutters

is often limited because of their scarceness and the presence

of metallurgical defects, in particular, of carbide inhomo-

geneity. Structural steels are used for such purposes with

thermochemical treatment of the surface. It can be found

in [1] that “... according to preliminary estimates nitriding

hardening should be applicable to carburized steels.” In this

connection we will consider the possibility of nitriding after

nitrocarburizing for hardening tools.

METHODS OF STUDY

We subjected some tools to low-temperature nitrocarbu-

rizing in a liquid carburizer (triethanolamine) [2], which did

not produce a carbide network in the diffusion layer, after

preliminarily testing the method on steels 20, 20Kh, and

20KhN3A. The nitriding was performed in an OK6 furnace

at 500 – 520°C with a hold of 40 h. The TCT was followed

by quenching, cold treatment, and tempering.

MILLING CUTTERS FROM STEEL 38Kh2MYuA

Milling cutters 24 mm thick and 100 mm in diameter

were produced from steel 38Kh2MYuA (GOST 4543–71).

The cutters for wood, specifically for milling slots in floor-

boards, were to have a hard core and strong, hard, and wear

resistant teeth. The cutters were nitrocarburized for 15 h at

800°C with triethanolamine fed at a rate of 65 drops�min and

then for 2 h at a rate of 45 drops�min. The nitriding was per-

formed for 15 h at 500°C and for 25 h at 520°C. Then the

cutters were subjected for scale-free hardening from 840°C

with cooling in oil, 2-h cold treatment at – 60°C, and low-

temperature tempering at 180 – 200°C. As a result, we ob-

tained a diffusion sublayer 0.81 mm thick, a top nitrided

layer 0.37 mm thick, and a 1 – 2-point carbide network (nu-

clei). The diffusion layer was not brittle.

The advantages of the double thermochemical treatment

relative to ordinary nitrocarburizing and heat treatment con-

sisted in the enhanced hardness of the diffusion layer (the

surface hardness was 895 HV and 759 HV respectively) and

the enhanced relative heat resistance (1.47 and 1.18 respec-

tively). The relative heat resistance was determined in terms

of the ratio of the hardness of the diffusion layer to the hard-

ness of the core before and after heating to 500°C. The dou-

ble TCT provided a higher wear resistance of specimens after

the diffusion layer was removed by the rotating abrasive pa-

per of a “Neris” machine.

The hardness of the surface layer of the cutters was

62 HRC at a core hardness of 51 – 53 HRC. The impact hard-

ness of specimens with diffusion layer over two adjacent

faces (without the concentrator slot) KCU = 0.50 –

0.56 MJ�m2. The cutters had a high strength in operation.

BLADES FROM STEEL 0KhN3MA

The blades of supershears 16 mm thick for cutting steel

bars up to 20 mm in diameter were produced from steel

0KhN3MA (TU 14-1-1S97–76) [3]. The shears were used at

room and negative temperatures. It addition to cutting bars

the shears performed a “compression-expansion” function

serving as a thruster. Some blades were subjected to 18-h

nitrocarburizing at 800°C with feeding of triethanolamine at

a rate of 65 drops�min and then for 2h at a rate of

45 drops�min (variant I ); the other part of blades was treated

for 18 h at 820°C and then for 2 h with feeding of triethanol-

Metal Science and Heat Treatment Vol. 45, Nos. 1 – 2, 2003

80026-0673/03/0102-0008$25.00 © 2003 Plenum Publishing Corporation

1 Elektrokhimpribor Works, Lesnoy, Sverdlovsk Region, Russia.

amine at a rate of 75 drops�min and 55 drops�min, respec-

tively (variant II ). After nitriding for 15 h at 500°C and for

25 h at 520°C the blades were heated in molten salt at

820 – 830°C and then hardened in oil. Then they were cold

treated for 2 h at – 60°C, and tempered at 180 – 200°C for

2 h and then at 500°C for 2 – 2.5 h with cooling in oil. This

treatment produced a diffusion sublayer 0.75 – 1.0 mm thick

and a top nitrided layer 0.3 – 0.6 mm thick. In this case the

carbide network was virtually absent (1 point), the observed

carbonitrides were 1 – 2 points in size (satisfactory), and the

layer was not brittle. The surface hardness was 53 – 56 HRC.

The core had the following mechanical properties:

�r = 1270 MPa, �0.2 = 1200 MPa, � = 11%, and � = 55%.

The impact toughness KCU of the core at 20°C was

0.88 MJ�m2; on specimens with a diffusion layer

KCU = 0.57 MJ�m2 on two faces; at – 60°C KCU = 0.67 and

0.27 MJ�m2, respectively.

Right after the heat treatment the blades were finished

for providing self-sharpening of the part with the diffusion

layer as described in [3]. The hardness of the “hard” face was

610 HV and that of the “soft” face was 547 – 587 HV at a

core hardness of 389 – 396 HV.

The blades were tested by cutting bars from steel 20 with

a diameter of 20 mm and showed satisfactory resistance at

– 60°C. After nitrocarburizing by variant I the blades were

used to cut 170 preforms; after treating by variant II they

withstood cutting 428 preforms.

REFERENCES

1. Yu. M. Lakhtin and Ya. D. Kogan, Nitriding of Steels [in Rus-

sian], Mashinostroenie, Moscow (1976).

2. L. P. Karpov, “Nitrocarburizing of steels with the use of tri-

ethanolamine,” Metalloved. Term. Obrab. Met., No. 2, 8 – 11

(1998).

3. L. P. Karpov, “Use of nitrocarburizing for providing self-shar-

pening of tools in the process of operation,” Metalloved. Term.

Obrab. Met., No. 6, 16 – 19 (1999).

Use of Double Thermochemical Treatment in the Production of Tools 9