Metal Sci('m'c and Ileut Treatment I-'ol. 41, Nos. 5 - 6. 1999

UDC 62 1,785.533:621.753.5

USE OF CARBONITRIDING FOR SELF-SHARPENING

OF TOOLS IN SERVICE

L. P. Karpov I

Translated from Metallovedenic i Termicheskaya Obrabotka Metallov, No. 6, pp. 16 - 19, June, 1999.

INTRODUCTION

Agricultural machines are equipped with sel l :sharpening cutting parts such as plough shares, shearers, and cultivator cutlers [I]. Sell:sharpening occurs due 1o the difference in lhe wear rates of the culling edges of the blade. The faces meeting at the blade edge have different hardnesses, the "'soft" |'ace wearing more than the "'hard" one. In order to in- crease the endurance of the blade the hardness of the "'hard" |ace is increased by various methods, including diffusion sat- uration [1 ]. The hardness of the "'soft" face is assumed to be a base constant. Optimum proportions lot the hardnesses of these two faces have been established [1] for specific opera- tional conditions. In machining metals the principle of self-sharpening is unknown.

The present work describes an attempt to realize the prin- ciple of selILsharpening in metal cutters.

METHODS OF STUDY

We used surface impregnation, in particular carboni- triding at 800~ in products of pyrolysis of triethanolamine developed earlier in [2, 3], for hardening the surface layers of cutters. The method -~ essentially consists of subjecting the laces of the blade to carbonitriding to a thickness of up to I ram, quenching, and tempering. The surface layer and the core of the blade are hardened. Then a part of the surfi~ce layer of the "'soft" face is ground off so that its hardness be- comes intermediate between that of the surface of the "'hard" face and the core. The diffusion layer of the "hard" face is not removed in order to preserve the maximum hardness, or is removed but to a much lesser degree than the "soft" lace.

We tested the cutters of guillotines and supershears from steel 40Kh containing 0.40% C, 0.63% Mn, 0.23% Si, and 0.80% Cr. The plate preforms for the blades were forged from rods 120 mm in diameter and then nonnalized. To make the blades self-sharpening we detemfined optimum regimes

I Elektrokhimpribor Works, Lesnoy, Sverdlovsk Region, Russia. 2 Positive decision on Application No. 97120523.

250

for grinding off the diffusion layer of the "soft" face and ob- taining a diffusion layer of the requisite quality under the condition of a high structural strength of the blades.

The thickness of the diffusion layer h and tile zone of carbonitrides (hn) was detemlincd metallographically, tile amount of carbonitrides (CN) was evaluated in points by the method of [4] (six points correspond to the coarsest carbonitrides), and the presence of a carbon itride net (CN N) in the layer was determined by GOST 8233-56.

The specimens were tested for fatigue strength using an INSTRON testing machine. The cracking capacity was eval- uated by indenting a Rockwell hardness meter into the stud- ied surface. The HRC hardness of the difli, sion layer was also measured.

The mechanical properties of the middle of the blades were evaluated by the results of tests of specimens subjected to carbonitriding and heat treatment together with the cutters. In order to eliminate saturation of the specimens their surface was copper coated.

The tests tbr tile impact strength were conducted for specimens having a diffusion layer on all faces and on two laces (a side one and a conjugate one without a notch con- centrator ~ (Fig. 1 )).

; Positive decision on Application No. 97109065 of 29.01.98.

Fig. I. Microscopic cross sections of impact specimens with a dif- fusion layer ( 3.6): a) over all faces: h ) over two thces.

002~5-0673/9~)/05(16-0250522.00 ~ 1999 Kluwcr Academic/Plenum Publishers

Use of Carbonitr iding for Self-Sharpening of Tools in Service 251

HJ'" 601). [

5o0

400

1,it I I).2 0.4 I).~ I).8 h, mm

Fig. 2. Distribution of the hardness over the thickness of the diffu- sion layer.

RESULTS AND THEIR DISCUSSION

Grinding Off the "sqli" /2we Of the blades. Supershears are used lbr shearing and cutting steel parts such as rods 21) mm in diameter. A plate cutter with a mass of 2 kg and a

thickness of 16 mm has a blade sharpened at an angle of 75 ~ The bearing face of the blade is "'hard" and presses the rod, leaving an impression on it. The "soft" lace incises the rod. Under the action of the two blades incising the metal in con- trary directions the preform is cut off(the rod ruptures trans- versely). The blades work with self-sharpening and with- stand over 500 operations of rod cutting under normal and negative temperatures (t >- 60~ The bluntness radius is

0.007 ram, The requisite depth of grinding the "'soft" tace is deter-

mined by the curve of the distribution of the hardness over the thickness of the difl'usion layer. In order to plot this curve the specimens or control blades are ground at a step of 0. I mm through the entire diffusion layer and the hardness is determined at each step (Fig. 2). The surface of the "'hard" (about 580 HI") lace is not ground, and the depth of grinding off the "soft" face is chosen by the corresponding hardness,

for example 470 - 530 HI! Such an approach to the formation of the cutting faces

of the blade al lowed us to increase their service life consi-

derably. Obtaining an optimum diJ.]itsion laveJ: The blades were

carbonitrided by different regimes (Table 1). The results of the study of the diffusion layer on the blades were used to choose the optimum saturation (regime 2 ). The technology of manufacturing the blades was supplemented by carbonitriding by regime 2, which provided an appropriate lower thickness (0 .85- 1.10 ram), not coarse carbonitrides (2 points, in some places at a distance of up to 0. I 74 mm from the surface their size corresponded to 4 points), and the absence of a carbide net (or the presence of only its initial features). The carbon content in the diffusion layer on the surface at a distance of up to 0.2 - 0.3 mm was 0.82% and that of nitrogen was 0.13% (0.01% in the middle).

The tests in an INSTRON machine with cyclic loading and indenting the cone of a Rockwell hardness meter into the

TABLE !

Carbonitriding regime h, mm hen, mm CN, CNN. points points

Double hold at 800~ for 15 h, v x=60 80drops/min(l)

800~ for 15 13 with v t = 65 drops/rain + 2 h with v t = 45 drops/rain (2)

0.87- 0.108 3 -5 I - "~ I.I0 0.105

0.85 - I).1)87 2 (up 1 2 1.10 0.174 1o4)

Notation: vt is the rate of feeding triethanolaminc: h is the thickness of the diffusion layer: hen is the thickness of the carbonitride zone: CN is the amount of carbonitrides: CNN is a carbide net.

surface of heat-treated specimens (Table 2) have shown that regime 2 chosen for carbonitriding is quite satisfactory. No fatigue cracks were discovered around the indentation.

The hardness over.the thickness of the diffusion layer de- creases monotonically, which makes it possible to predict the depth of grinding off the "'soft" face.

Structural strength. In order to provide the requisite structural strength of se l f sharpening blades we detemfined the regimes of quenching and double tempering after the carbonitriding.

In order to preserve nitrogen in the diffusion layer the austenization temperature (800 820~ for the subsequent heat treatment was chosen below the standard level. The variation of the temperature (800, 820, 840~ changed the hardness of the diffusion layer alter quenching in oil and tempering at 400~ with cooling in the oil unsubstantially (52 - 54 HRQ) , and that of the core also (42 - 45 HRC.). The impact toughness of the middle K('L +=1).65- 0.66 M J/m2: in specimens with a diffusion layer on two

"[ABLE 2

Carbonim- P, N N h,, d .mm her ding regime

1 3000 500 0.06 0/0

10,000 300 0.54 I/'-" 2

15,000 200 0.66 I/2

10,000 1000 0.36 0/It

2 3000 500 0.19 1)/1

10,000 300 0.40 Off)

15.000 200 0.53 I)/2

I 0,000 1000 0.42 0/0

Notes. I. We give the results of evaluating the resistance to contact fatigue for specimens subjected to cyclic tests under a IoadP with number of cycles N. 2. The numera- tors give the number of cracks her around the indentation on the surface of the specimen: the denominators give the number of cracks on conical indentation left by a Rock- well cone thin d is the indentation depthl.

252 L. E Karpov

faces, KCU = 0.21 0.24 MJ/m 2. The strength and ductility of the middle fluctuated within the lbllowing ranges:

cYo2= 1350- 1400MPa, cyo.z/Cy~-0.964-0.079, 6= 11- 13%, and tl/= 42 - 48%.

A low-temperature tempering at 180-200~ is con- ducted between quenching and tempering at 400~ which is connected with the special features of the manufacturing pro- cess of the tools. Each blade is quenched separately and a batch of blades is formed for subsequent tempering at 400~ In order to eliminate cracking of the blades in aging before the tempering at 400~ they are subjected to a low-tempera- ture tempering right after the quenching.

The temperature of 400~ is optimum for the second tempering because it provides a satisfactory toughness of the middle and the surface layer of the blades and a sufficient hardness of the diffusion layer. Low-tempered blades frac- ture by a brittle mechanism when tested. A similar techno- logy for carbonitriding and heat treatment 4 is used tbr guillo- tine blades from steel 40Kh. The thickness of the blades is 20 mm and the length is up to 635 mm. Short blades are sub- jected to light quenching: long blades are quenched after heating in a chamber thrnace to 840~ Alter carbonitriding and heat treatment the surface layer of the blades is ground o f fby 0.2 mm C'hard" face) and 0.4 mm ("soft" lace). After the grinding the hardness of the laces is 492 and 432 HI" re- spectively. The endurance of pilot blades is 1,5 times higher than that of blades produced from tool steel with the same high hardness of the cutting faces,

4 Patent No. 2117069.

CONCLUSIONS

1. Carbonitriding at 800~ and heat treatment of flat

blades of supershears and guillotines from steel 40Kh with

subsequent grinding of the faces to a specific depth provides

sell-sharpening of the blades in operation. 2. In order to provide a self-sharpening regime alter the

creation of the diffusion layer on the faces of a blade and a

hardening heat treatment, a part of the layer of the "'soft" Face

is ground off to a hardness much lower than that of the sur-

['ace of the "'hard" face. 3. The structural strength of the blades is provided by op-

timum regimes of carbonitriding, quenching: and double

tempering and raising the toughness of the "soft" face by a

partial removal of the surface "'hard" layer.

REFERENCES

I. V. N. Tkachev, B. M. Fishtein, V. D. Vlasenko, and V. A. Ulanov. Methods/or Increasing the Service L(/e 0/ Machine Parts [in Russian]. Mashinostroenie, Moscow (1971).

2. k. R Karpov, "'Carbonitriding with the use of triethanolaminc at 800~ Metalloved. Term. Ohrah. Met., No. 2, 8 - 12 (1998).

3..4 Method.fin Carbon#riding Steel. RF Patent No. 208282{). 27.06.97 [in Russian].

4. V. D. Kal'ner (ed.), Control over the Quali O' q/lleat T/eatmenl q[ Steel Semit)roducts and Parts. .q Handhook [in Russian]. Mashinostroenie, Moscow (1984).