Boronizing and its Practical Applications · Boronizing paste ~) The state of boronizing technology Boronizing powder, granules and paste With a relatively new process, such as this

Boronizing

and its Practical Applications

Walter Fichtl

Elektroschmelzwerk Kempten GmbH, D-5020 Frechen

The research study "Tribologie " (tribology) published by the Federal Ministry for Research and Technology 1 reports that in the Federal Republic o f Germany alone, the losses to the national economy due to abrasion and wear amounted to more than 10 billion DM in the year 1975, i.e. approx. 1% o f the gross national product. It is therefore understandable that increasing measures are being taken worldwide to combat wear. In the battle against wear, EKabor boronizing agents have proved to be very effective in industrial practicefor some years, and one can safely say that boronizing is already regarded as one of the conventional methods o f surface hardening 2. Above all, the process makes a significant contri- bution to combating wear in extreme conditions.

Fundamental principles of the process General Data

Boronizing is, like any other surface hardening process, a thermal diffusion of boron into the surface layer of a workpiece by thermo- chemical treatment. In the process, boron atoms are introduced into the metal lattice at the surface of a work-piece through thermal energy to form borides with the atoms of the substrate.When iron, the most current base metal, is boronized, iron boride will result. A picture

taken with a scanning electron micro- scope (Fig. 1) shows dentiform iron boride crystals (Fe2B) of roughly 120 microns length; here the iron substrate was eliminated by boiling the piece in 19% hydrochloric acid for several hours. The phase diagram of the binary system iron/boron 4 (Fig. 2) shows the presence of two compounds, namely Fe2B with 8,83 wt% boron and FeB with 16,23 wt% boron, as well as an eutectic with 3,8 wt% boron and a melting point of 1 149°C (1 422 K).

Modifications o f the process The processes known so far are

based on chemical or electro-chemical reactions between the boron source and the respective base metal. Potential boron yielding materials are boron compounds of any of the three physical forms.

To work with gaseous boronizing compounds requires complex equipment - although the process is, in itself, an ingenious, yet simple one. But there are, in addition, two further disadvantages:

Fig. 1 Iron boride layer (Fe 2) in which the matrix (Ck 45) was dissolved using 18% hydrochloric acid

MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981 276

1600

1400

.~ 1200

8

1000 I---

~3t,'C == -1550'C

I\ 1 , f o 142e'c m1"c ,1,,, c _ ~ ( 1 ' 5 ' A - ~

¥ II.~ Ul I135*C 0,1 -17

70,02) (3.8)

910-* 5'C

t..L

800 q-

I L I I 600 0 20 40 60

B0r in Atom-*/, I I [ I I I I I J l I I 0 2 4 6 B 10 20

Bor in Gew-%

Fig. 2 Bin arY4system iron-boron according to .

1. Diborane, the main compound used, is extremely toxic. The lower toxic limit for Diborane is 0,1 ppm, compared with the 10 ppm allowed for the extremely dangerous hydrogen cyanide! (prussic acid).

2. Even when diluted by hydrogen, Diborane is still very expensive. Liquid boronizing compounds are

used either as an electrolyte or in immersion processes. Various problems of a technical nature, tied up with the process as well as the fact that a biphase boride zone consisting of Fe2B and FeB is mainly produced, have been an obstacle to the large scale introduction and use of liquid boronizing compounds.

For these reasons the solid boronizing compounds are of primary importance. Their essential benefit is that their use involves little equipment. The process may be carried out practically in any tempering shop that is equipped with box furnaces for case hardening operations; the following materials are currently available as a boron source for solid boronizing compounds; amorphous boron, ferroboron and boron carbide. Amorphous boron is very expensive (approx. DM 250, - per kg), whereas ferroboron (approx. DM 7, - per kg) can be produced neither in commercial quantities nor to the required degree of purity. For practical application only boron carbide is available of unvarying quality, and at a comparatively low price (approx. DM 40, - per kg).

With powdered boronizing compounds based on activated boron carbide - one can produce simply and comparatively cheaply boride zones on steels and ferro alloys, whereas with the other boride mixtures, only at comparatively low temperatures (approx. 800°C/= 1 073 K) and with short treatment) mostly 2 to 3 hours), could single-phase zones - i.e. free from FeB - of a maximum thickness of approx. 40 microns be obtained. Thicker zones contained inevitably more than 50% of the second FeB phase and were highly prone to cracking and scaling as a result of large differences in their inherent stress levels, (Fe2B = compressive strain/ FeB = tensile stress).

D e v e l o p m e n t of new boroniz ing c o m p o u n d s

During the past 10 years ESK has succeeded in developing formulations for the production on a large scale, of single-phase boride zones consisting of Fe2 B s-9 with an average thickness of 10 to 300 microns; these single-phase Fe2 B zones can be detected metallographicaUy. For extreme erosive stress, zones of 800 to 1000 microns have already been produced using special formulations. Furthermore, there is a special boronizing paste on the market for specific applications, such as boronizing of produced parts and/or selective boronizing. With this paste high grade economic boronizing is possible as will be explained in paragraph 4.

deformation of the substrate and simultaneous destruction of the iron boride zone.

Acid resistance It has been a known fact for some

time that the acid resistant properties of low alloyed ferrous material may be considerably improved by boron- zing ~°. Tests carried out by the author s not only corroborated this statement (Fig. 3)but also showed that boronized austenitic steels are much more resistant to hydrochloric acid than non boronized ones (Fig. 4).

This experience has led to the successful application of the steels in chemical technology, as will be described in paragraph 4.

L

/ / ;

/

/ '

/ / , , . .

Fig. 3

Propert ies of the bor ide zones Micro hardness

The micro hardness (HV or HK 0.025) of the bodde zones on commercial iron materials lies roughly between 18000 and 21000 N/mm 2 thus being in the range of corundum ~ hardness. Occasionally values up to ~i 24000 N/ram 2 HK 0.025 are measured on high alloy steels.

The micro hardness test must be ~ ~0 carried out either on polished and etched microsections, at right angle to the surface of the specimen, or on the surface of a work piece that has been polished perfectly after boronizing.

With the Vickers or Knoop methods greater test loads mostly lead to faulty results (cracking and scalinga). Fig. 4 The Rockwell and Brinell test methods are not at all suitable for measuring boride zones, as their use involves

Loss in weight of a C k 45 steel in mineral acids 56°C (329 K) according to 8.

100

' I , V t 2~;. Hct i / r I

- - o o ntcht b0rlerl - ~ - - - ~ - - e-...--.--e oor,ert / ! /

, i / t t

/ t E ! i / + ! I

[ + - - - - - + mchI b0rlert I x . . . . "7× bor e t /,,

: i i ,

Do--" ? - - - - ~ 6 P

Loss in weight of 321 S 12 (En 58B, En 58C) - DIN No. 1.4541 in 20% HCl and 10% H2SO 4 ~t 56°C (329 K) according to

277 MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981

Table 1 Boronizing Agents

Type Grain Size ,um

Density, g/cm 3 compacted

Boronizing powder I a) L 150 Boronizing powder 2 ~) _~ 850 Boronizing powder "~) ~ 1.400 Boronizing powder HM ~) _~ 150

(for carbides)

1.9 1.7 0.95 0.95

Boronizing paste ~)

The state of boronizing technology Boronizing powder, granules and paste

With a relatively new process, such as this one, many new developments come from user requirements. Table 1 lists the extensive range of boronizing agents.

Precision boronizing, i.e. boronizing parts with close dimensional tolerances, should be carried out with the boronizing powders EKabor 1 and 2. These powders can be re-used up to six times if the mixture is replenished by the addition of 20-50% of fresh boronizing powder, the amount depending on the preceding boronizing conditions.

Boronizing powder EKabor 3 is used preferably for boronizing parts for contractors' plant etc.

Boronizing agent EKabor HM has the greatest boronizing action. It was developed particularly for the treatment of tough carbides, high alloy steels, and small bores in low-alloy or non-alloy steels. Boronizing (complete or partial) of mass-produced parts or of workpieces of complex shape is carried out most economically using the boronizing paste EKabor. In the current state of development, however, an inert gas atmosphere or vacuum is then necessary i 1-14. The action of the boronizing agent is greatly influenced by the properties of the gas 14 , and in many cases the best results can be obtained by using argon 1 a.

Further EKabor products for special purposes are under development.

Materials suitable for treatment These include most constructional,

case hardening, tempering, tool and chemically stable, wrought steels as well as cast steel, Armco ingot iron, most cast iron grades (among others gray and spheroidal graphite cast iron), sintered powder metals and sintered steel ~ s-~ 6

for boronizing mass-produced parts and partial bcworiizing (v,,ith inert gas)

Table 22 shows a list of steel types that have successfully been boronized.

Another benefit of boronizing is that, for parts requiring only a high surface hardness, low alloyed (boronized) grades can now be used instead of the more expensive high alloy steels that are difficult to work.

However, steels containing alum- inium should not be boronized, e.g. nitriding steel (34 Cr A1 Ni 7 - material no. 1.8550), also, steels with a Si content from about 1 wt% are an inappropriate substrate for thick boride zones, as both these metals will be pushed back by the boron diffusing into the surface to settle below the Fe2B phase in the diffusion zone producing ferrite there. As a result the extremely hard boride zone will be anchored to a zone that is still softer than the original substrate material (Fig. 5). If work-pieces having such a structure are exposed to strong surface loads an undesired phenomenon will occur in that the extremely hard, yet - due to its thickness - brittle boride zone will be pressed into the very soft inter- mediate zone and thus be destroyed.

Other metals that are suitable for boronizing are e.g. cobalt and nickel and any hard metals containing these elements in their substrate. According to the results available to date hard metals should, to be satisfactory, only be boronized when their cobalt or nickel content is/> 6 vol%.

Copper cannot be boronized, but it is successfully used as a masking agent for selective boronizing, in the form of thin self-adhesive films or sheets etc.

Operating conditions Temperatures and treatment time

If the substrate material is suitable temperatures ranging from 800 to

i .

Fig. 5 Steel with 1,5% Si, for 12 hours, at 1.050°C (1.323 K), EKabor 1, ferritie zone be low the boride layer - the impressions of the Knoop-diamond show the following mierohardness values:

Fe2B 1.800 HK 0,1 ferritic zone 340 " matrix 570 "

1050°C (1073 - 1323 K) can be used, except for gray cast iron and hard metals. In the case of gray cast iron temperatures must not exceed approx. 850 to 880°C (1123 - 1153 K) since the phosphide eutectic ("Steadite") contained in gray cast iron will melt at 950°C (1223 K), with the result that there is a danger that the surface of the workpiece will be deformed. The maximum temperature advised for hard metals is around 9000C (1173 K), at higher temperatures besides boronizing of the substrate metal itself- such as cobalt, nickel or iron - a conversion would occur of the carbides of tungsten or, possibly, titanium/tantalum or tantalum to form the respective borides.

The treatment time ranges from 15 min. (e.g. for small parts in rotary retort type furnaces) to 30 hrs (for parts exposed to arduous duty). Normal treatment time is to date 1 to 8 hrs. Also it must be noted that the combination of high temperatures + short treatment time (Fig. 6) should be preferred to low temperatures + long treatment time.

Subsequent heat treatment Since the coefficient of thermal expansion of 14 x 10 -6 X degree -1 of the boride zone lies roughly in the


I I I I I L ~ | mox~m 5chichldtcke I , ~ I - _ -

E 300 I'~ - - ~ m l l t l e r e S c h l c h l d i c k e ~ ==- I (M*tlel der IIIngsien ~ ~'~

Fig. 6 Dependence of the boride layer thickness on treatment temperature and time in a Ck 45 steel according to 8.

centre of the alpha range of the normal commercial ferrous alloys or steels the core strength can be improved by conventionally quenching the parts either in a warm bath, in oil or in air after boronizing, followed by tempering. Hence quenching and tempering will normally not lead to cracking of boride zones up to 150~m, if the process is carried out properly. Yet it is imperative to execute austenising and tempering in a neutral environ- ment, preferably with a protective atmosphere or a neutral salt bath or under vacuum.

Boronizing and austenising may also be executed simultaneously, provided that the size of the workpieces requires similar treatment times. In such cases it is advisable to use either boronizing granules (EKabor 2 or 3) or EKabor paste.

Optimum thickness of boride layer It has long been known that the

optimum boride layer is not necessarily the thickest possible layer. The thickness should always be matched to the intended application. This means: Thick layers for erosive wear, (for example: in the extrusion of thermo- plastic and thermosetting materials with high abrasive Idlers such as glass or asbestos fibres and wood flour or pigments such as titanium dioxide), and thin layers for adhesive wear.

In theory, boride layers approx. 5 #m thick would already be adequate to prevent adhesive wear. However, because of the interlocking nature of the Fe2 B crystaUites, it is not possible to produce even layers of this thickness on non-alloy or low-alloy steels. For such purposes, e.g. for tools used for chipless forming of metals, it has been found that the best results are obtained with high-alloy steels having layers approx. 15 to 20 /am thick (Fig. 7). The fact that a two-phase boride layer, i.e. consisting of Fe2 B +

FeB, is involved is of no importance for the range of thickness mentioned ~ 7

Therefore it can be concluded that the choice of the base material is determined by the intended use,

bearing in mind that the diffusion of boron into steel becomes increasingly difficult as the alloy content increases.

For economic considerations it is not possible to produce and stock a

Fig. 7 Fe2B + FeB on AISI 316 Ti steel (DIN 1.4571), 4 hrs/900°C (1.173 K)/EKabor 1

a)

t f

~ t~ ~ I d;• @, t o, i • e t

4

b)

Fig. 8 AISI 316 Ti steel (DIN No. 1.4571), boronized in EKabor 1 at 900°C (1.173 K) for 6 hours, according to 2.

a) no after-treatment b) at L000°C (1.273 K) for 2 hours - in argon


Table 2 Examples o f materials

Material

AISI

" 102()

1043

U 113S

~" 1042

D 3

L2

H l l

H I 3

H l(I

D 6

S I

D 2

_ 6

O 2

-2 52 l l i l i

410

420

BSI

BD 3

B H I I

P,H 13

BH Ill BH I l i a

BS 1

Nr. 5 __4 (FIN "> )

B() 2

410 S 21

420 S 45 (f/.n 56 D)

3(12 3o2 S 25 (En 5SA)

316 - 31~ S 16 ( Un 5S .I )

321 321 S 12 (t-nSSB, t{n58(')

316 Yi

4317

4140 71)g A 42 (En 19C)

4150 - 7(t8 A 42 ( C D S - 15)

cast iron

and appl icat ions

Application

1.0401 bushes, pipe bends, conveyor tubes, baffle plates, runner, base plates, heli- cal gear drive for oil pumps, pump shafts

1.0503

1.0530

pins. guide rings, grinding disks, bolts

castings inserts, nozzles (jets)

1.0727 shaft protection sleeves, mandrels

1.1191 swirl elements, nozzles (for oil bur- nets), rollers, bolts

1.2()SO press tools, punches, die,,

1.221() dra~ing punches

1.2343

1.2344

phmgcrs, injection cylinders, spruc orifices, ingot moulds

1.23,'}5 forging dies

1.243~, ~traightcning rollers

1.2550 dies. necking rings, punches, dra~ing die',

1.2001 clra~ing tools, rolls for c,~ld mills

1.2714 bolts, casting inserts, drop forges, lorging tiles

1.2S42 bending dies, moukts, engraving rol- Icrs. drav, ing dies, press tools, pierc- ing punches

13~( "~ balls and rollers for rolling bearings

1.4(i()6 ~al~e components

1.4034 parts for chemical plants

1.43()O screw cases, box screws

1 44()1 1.44 II)

perforated screens, valve plugs

1.4541 rings, conveyor jets, injectors

1,4571 parts for the chemical industry

1.6587 bevel gears

1.7225

1.7__~

press tool dies. extruder screws, extruder barrels, ram-return valves

nozzle base plates

parts for textile machines, sleeves, moulds

wide range of boronizing agents to meet every special application, i.e. there is no boronizing agent which can produce a single-phase boride layer in every case 2 . In addition to the repeat- edly mentioned factors of base material and boronizing temperature and time, the rate of heating, particularly in the region between approx. 700 and 800°C (973 and 1.073 K), plays a very important role. These are the reasons: The diffusion rate of boron atoms, even in the case of non-alloy or low-alloy base materials, only reaches a magnitude adequate for the production of technically serviceable boride layers at temperatures above approx. 800°C (1 073 K). On the other hand, already above 700°C (973 K), a continuous supply of active boron is produced by thermo-chemical reaction. Therefore unless the above mentioned temperature range is quickly traversed by rapid heating, boron builds up in the outer region, so that during this stage the second FeB phase is formed, which can often be troublesome in thick layers. Experience shows that this layer cannot be eliminated during further boronizing.

Investigations on the removal of FeB in high-alloy steels

For special purposes, such as simultaneous erosive and corrosive wear, the chemical industry requires high-alloy steels with well adhering boride layers, more than 20grn thick. The following process was presented as a solution to this problem:

1. Production of a relatively thick, two-phase layer, C~ 30/am)

2. Subsequent diffusion in an inert medium,, preferably in an inert gas atmosphere. Based on a series of tests under

varying conditions the optimum for an 18% Cr 10% NiMoTi steel (type DIN No. 1.4571) 2 were found as: For 1: Boronizing for 6 hours at

900°C (1 173 K) in Boron- izing powder EKabor 1 ;

For2: Subsequent diffusion for 2 hours at 1 O00°C (1 273 K) in argon.

The specimens treated in this manner are shown in Fig. 8. It was possible to transform the original duplex layer, consisting of 20/am FeB + lOoxn F%B, into a single-phase one approx. 36/am thick layer by means of the subsequent heat treatment. This means that in addition to a more homogeneous layer less prone to crack

M A T E R I A L S IN ENGINEERING, Vol. 2, DECEMBER 1981 280

formation and flaking, an increase in thickness of 20% is produced. In practice the pores formed in the region of the original FeB phase do not have a detrimental effect.

Successful applications Table 2 shows typical combinations

of materials and types of parts, which have proved particularly successful in the boronized form 2. A few of the numerous engineering applications will be discussed in greater detail below:

Because of the severe erosion caused by particles of minerals adhering to tobacco leaves, perforated strips made from 17% Cr stainless steel (DIN No. 1.4016) Fig. 9 had to be replaced usually every four weeks. When boronized at 880uC (1 153 K) for five hours in EKabor 1 or 2 powders, the service life was increased to two years, i.e. 25 times that previously obtained.

hours in powder EKabor 1 or 2. The life of boronized coffee grinder

disks for commercial coffee roasting plants is five times as great as that of nonboronized disks. The materials is 0.45% C steel (DIN No. 1.0503), boronized for four hours at 850°C (1 123 K) in EKabor 2 powder.

Because of the very good results achieved, boronizing is now also being used regularly for cast iron textile machinery parts (Fig. 11). Boron- izing is carried out at 820°C (1 093 K) for 6 hours in powder EKabor 1 or 2.

The four holes in a feed water regulating valve (Fig. 12), made from 18% Cr, 10% NiMoTi steel (DIN No. 1.4571) and used in a large chemical plant, normally became a single slit after 2 000 operating hours due to erosion. SteUite coated parts lasted for a maximum of approx. 8 000 operating hours, whereupon wear was so advanced, that, even with the valve

Fig, 9 Perforated strip of 17% Cr stainless steel (DIN No. 1.4016) (775 mm long), 5 hrs/880°C (1.153 K)/EKabor 1 or 2

Fig. 10 shows the upper and lower moulds for producing ceramic components. The life of these moulds of 0.9% C 2.0% MeCrV steel (DIN No. 1.2842) was almost trebled by boronizing at 900°C (1 173 K) for five

closed, 50-60% of the maximum flow still occurred. If the regulating valve is made of 13% Cr steel (DIN No. 1.4006), boronized for 6 hours at 900°C (1 173 K) in EKabor 1 or 2 powders, and subsequently subjected to appro-

Fig, 10 Moulds for ceramic parts of 0,9% C, 2% Mn Cr V steel (Din No. 12842), 5 hrs/ 900°C (1.173 K)]EKabor 1 or 2

Fig. 11 Part for textile machine, cast iron, 6 hrs/820°C (1.093 K)/ EKabor 1 or 2

priate heat treatment, then it is still fully operational after 18 000 hours I s

Fig. 12 Feed water regulating valve of 13% Cr steel (DIN No. 1A006), 6 hrs/900°C (1.173 K)/ EKabor 1 or2

Pneumatic conveyor elements for the chemical industry have been boronized in series for several years. These elements are subjected to very severe erosive wear through the action of glass fibre reinforced plastic granules.

Formerly 90 ° bends of 100 mm diameter and their relevant baffle plates were made of austenitic Cr Ni steels; their maximum life was about 6 weeks, boronized bends, however (6hours/1 000°C = 1 273 K/EKabor 2) of non-alloy steel (St 37) had 4 to 6 fold the lifetime, under comparable conditions.

Reducing bends (diameter 400 to 250 ram) weighing roughly 500 kgs each had to be boronized on the inner surface (Fig. 13). However, in view of the irregular diameter and shape of the bends it was not possible to save boronizing material by using inserted iron tubes. Therefore, the inner side was prepared twice with boronizing paste. After drying of the paste boronizing was carried out in an inert gas atmospher (I0 hours/950°C = 1 223 K/Argon). The costs were about DM 3 000, - per piece, whereas powder boronizing would cost about DM 5 000, per piece because of the difficult geometric shape. This means 40% cost saving s 9.

Another field in which boronizing has been successfully applied for years


Fig. 13 Reducing bend (nominal with

is that of burner nozzles, swirl elements and injector tops (Fig. 14), both for industrial oil firing and in plants for the disposal of liquid chemical wastes in the chemical industry. In these industries, heating oil, steam and waste are forced through the

Fig,. 14 Oil burner parts made either of 0,45% C or 18% Cr 10% Ni Mo Ti steel (DIN Nos. 1.1191/1.4571)

400/250), steel no. St 37 according to 19

nozzles at pressures up to g bar. In most cases, the parts mentioned are made of low P and S, 0.45% C steel (DIN No. 1.1191), boronized for 6 hours at a temperature of 900°C (1 173 K) in EKabor 2 powder and achieve a service life approx. 2-3 times greater than the previously employed parts. Only in special cases e.g. for the disposal of liquids containing prussic acid, 18% Cr 10% NiMoTi steel (DIN No. 1. 4571) boronized for 6 hours at 850°C (1 123 K) in EKabor 2 powder must be used to achieve a similar 2-3 fold increase in service life.

Dies (Fig. 15) for making pipe clips from severely scaled rolled steel were previously made in heat treated 0.9% C, 2% McCrV steel (DIN No.

Fig. 15 Die for pipe clips of 0,9% C 2% mn Cr V steel (DIN No. 1.2842), 6 hrs/900°C (1.173 K)/ EKabor 1

1.2842) which was subsequently hard chromed. Such dies were worn out after the production of 10 000 parts. If the same material is boronized at 900°C (1 173 K) for 6 hours in EKabor 1 powder and is subsequently heat treated, then the tool exhibits after 17 000 parts a highly polished surface and still conforms to the required dimensions.

/ ' /" / / fs" / /

Fig. 16 Pawls of 0,7% C steel (DIN No. -1.1249)

Fig. 16 shows four different designs of pawls for counters. The material is 0.7% C steel (DIN No. 1. 1249), boronized for 3 hours at 900°C (1 173 K) in EKabor 1 or 2 powder. The pawls already show a substantial increase in service life, although final figures are not yet available.

Fig. 17 Bogie suspension part of 3% Ni 1% Cr 0,5% Mo steel (DIN No. 1.6746)

Suspension parts for railway carriage bogies (Fig. 17) made of 3% Ni, 1.0% Cr, 0.5% Mo steel (DIN No. 1. 6746) are usually worn out after 100 000 km. Replacing these parts


is very expensive, as the entire carriage has to be rifted off. Suspension parts boronized for 4 hours at 900°C (1 173 K) in EKabor powder are still usable after 200 000 km.

In West Germany, large scale boronizing was first applied to parts for high performance internal combustion engines. Since then, boronized driving gears for the oil pump in the Diesel engine for the Volkswagen Golf car (Fig. 18) have proved very successful in service 2°. These boronized gears are a standard production item ever since this version of the car has been produced. These two gears rotate at peripheral speeds up to 5.6 m/see.

Fig. 18 Driving gears for VW Golf Diesel engine

The aim of designers of compact cars is to pack a maximum of engineering into relatively little space. In this case~ the skew gears were arranged at an angle of 110 ° relative to each other. The tooth load is therefore appreciably higher than in the case of a right angle drive. Due to the potential high adhesive wear on the tooth flanks, a process had to be found which, in conjunction with a suitable base material, could withstand such conditions over a prolonged period. Boronizing and subsequent heat treatment of a 1% CrMo steel is the optimum solution for this application. The manufacturer reports that skew gears treated in this manner are practically free from wear.

In cutting tools two different types of wear can be stated: when processing metals the tools will be cold welded (adhesive wear), in the case of non-metallic materials (e.g. plastics with Idlers) erosion will occur. Both phenomena can largely be eliminated by using boronized tools.

In punching dies the boride layer is prone to shatter at the cutting edges. Consequently, in practice, only the cylindrical shaft is boronized, over a length of about 20 to 30 mm, beginning at 0.1 mm above the cutting edge. Fig. 19a shows such a punch-

I

Fig. 19 a) punching die,DIN No. 1.2601, according to 21

b) boride layer on a punching die of a 1.2601 steel

ing die made of 1.65% C, CrMoV steel (DIN No. 1.2601). Boronizing and subsequent heat treatment are made in a single operation in a vacuum furnace using EKabor paste. The boride layer is about 30t.tm thick (Fig. 19b), the hardness of the core is 60 to 62 Rockwell C. The original diameter of 6 mm increases in a reproducable manner by only 3/.tm, that is 0.5% 2 ~.

According to H. Kishimoto 22, boronized forging dies in a wide range of sizes have been used successfully in Japan since 1972. The forging dies are made of 0.4% C, 5% Cr, 1% Mo, 1% V steel ("SKD 61"), boronized for 3 hours at 900°C (1 173 K) in EKabor 1. The dies are subsequently air hardened and tempered. The average service life is increased six fold by boronizing, the minimum being 2 or 3 fold and the maximum ten fold.

In West Germany, promising tests have also been in progress for some time with boronized forging dies made of 0.3% C 3% Cr, 3% MoV steel (DIN No. 1.2365). This work is being carried out under the direction of the Research Department for Forging Dies at the Technical University of Hannover. H.G. Joost has reported on the preliminary results 23 . Tests were carried out on an automatic crank press (rated capacity 3.15 kN) in the Research Department for Forging Dies. Specimens of 0.3% C, 1.0% Cr steel were heated to 1 100°C (1 373 K) and forged with a single blow of the press. The force at the end of the blow amounted to approx. 600 N/mm 2 .

The entire impression in the die, including the flash gap, was boronized. After every 300 forging operations some workpieces were heated in an inert atmosphere, quenched in oil, and checked for wear on the boss nose radius (radius 3 in Fig. 20).

~b150

' 1 2 3 I I j I

! r J j ~ ' , l ~ t?

[ ~ . / / J ; ; / . ~ ; J C / , / / . , J: l ~ [ ) i

Fig. 20 Forging die o f 0,3% C 3% Cr 3% Mo V steel (DIN No. 1.2365) according to 23 and 2

In contrast to forging dies with other surface treatments, the boronized forging dies showed no measurable wear (Fig. 21). The tests are being continued at present on a range of production forging dies, which entail different forging loads. Amongst similar results are those obtained by S.I. Gorerik 24 who found that the life of hot forging dies was increased 2-3 fold. H. Zoellner 2s reported on applications for other common mass production process.

200F~~~~ C a50 -T,N-

_~ 100 . . . . . '

> s 0 1 - - U 2 " i , - i

Borleren Borle en 7

0 300 900 1500

Anzahl Schrniedestucke Fig. 21 Wear on boss nose radius,

according to 23 and 2

The number of load cycles with- stood by the clamping jaws for mech- anical testing machines is increased appreciably when they are boronized. In this case the astonishing fact is that, despite the very high tensile and compressive loads encount- ered in the destructive testing of materials, a breaking away of the boride layer in the region of the clamping jaw serrations does not occur. The original saw tooth profile was modified to a trapezoidal profile, which is appreciably simpler to produce and, above all, can be incorporated before heat treatment.

Apart from saving production costs, material costs were also reduced. High alloy tool steels (e.g. 2.1% C, 12% Cr) were replaced by simple heat treated materials such as 1% CrMo or 1% CrV


steels. The boronizing of these steels is carried out in EKabor at 900°C (1 173 K) with a soaking period of 4 to 5 hours. The boronized clamping jaws were subsequently heat treated to give sufficient core strength to prevent indentation of the base material by the boride layer under high compressive stress. In this case austenising is carried out in a neutral (cyanide free) salt bath with subsequent quenching in oil.

The pipes carrying vinyl chloride in a plant producing PVC 26 are made from 18% Cr, 9% NiTi and 18% Cr, 10% NiMoTi steels. A section through a part of the pressure and vacuum lines is shown in Fig. 22. Prior to boronizing, this part had a life of only half a year. VC gas, having a temperature of 70 to 80°C (343-353 K) and a pressure ranging from 0.067 to 12, bar flows through the pipe bend shown. The cause of the premature wear was that vinyl chloride contained traces of hydrochlorid acid, which, in conjunction with moisture (steam injection), had an aggressive action.

Since the internal surface has been boronized, the life of the pipings has been increased to more than two years, despite the fact that the boride layer is only 15/am thick.

Fig. 22 Pipe bend (DIN No. 1.4571 according to 25-26

Owing to the high alloy material used, the heat treatment is carried out in EKabor 2 at 850°C (1 123 K) for a period of 4 hours. Even after two years, microscopic examination of the boronized, the life of the piping has the grain structure by corrosive attack. Some boronized components, such as extruder screws and barrels,

have already proved successful in the plastic processing industry, as has been reported extensively in the literature.

Other boronized parts, which have already been in regular use for an appreciable time are non-return valves. The life of nitrided non-return valves used in the processing of elastomers containing silicon frequently lasted only 5 to 8 days. Nowadays, entire non-return valves are made from heat treated 1% CrMo steel, boronized at 950°C (1 273 K), in EKabor 2 for 5 hours. Heat treatment in vacuum is carried out subsequently to increase the core strength. The life of the non- return valves treated in this manner is on an average 8 times the previous life.

For cutting up plastic strands into granules of the required size the chemical industry uses special equipment. The rotors and impact cutters are subjected to severe wear, due to the high rotor speed of 3 000 rpm and the abrasive granules.

The specification of the granules produced requires the use of high alloy, corrosion resistant materials for the parts, e.g. 18% Cr, 9% NiTi steel. Experience has shown that these steels are not basically very abrasion resistant, so that a process had to be found, which is not detrimental to the good corrosion resistant properties of the base material, but provides high resistance to abrasion.

As the company concerned already had several years of good experience with the use of other boronized components (e.g. pipe bends and baffle plates for pneumatic con- veyors), the boronizing process was used to solve the new wear problem. The rotors and impact cutters made from 18% Cr, 9% NiTi steel are boronized at 850°C (1 123 K) for 6 hours. This trebled the life compared with untreated parts.

Boronized and subsequently heat treated steel cores are used to form the cast cylinder bore and the combustion chamber of two-stroke" engine light alloy cylinder blocks produced by gravity die casting. Compared with unboronized cores, the wear at the edges of the steel cores was reduced appreciably, so that their life is about 5 or 6 times that of the previous life.

The 1.75% CrMnMo steel used up to now has been found suitable for boronizing. EKabor 2 is used as boronizing agent in conjunction with a soaking time of 5 to 6 hours at 850°C (1 123 K).

The core of the steering drop arm (Fig. 23) made of 1.25% MnSi steel is produced as follows: 1. Drilling 2. Broaching 3. Sizing of the serrations (48 teeth) 4. Inspection of size and shape.

The sizing of the serrations is carried out with a tool made from 2.1% C, 12% Cr tool steel. Up to now, this tool, hardened to 63 to 64 Rockwell C, could produce up to 200 of the drop arms, which have a tensile strength of 800 to 950 N/mm 2. This meant relatively high tooling costs for a mass produced part, such as this drop arm.

" / / /

~ . . . . i ̧

, ~ ® 4 5 . 5 H 1 0 -

Fig. 23 Steering drop arm, steel DIN No. 1.5122, worked with a mandrel of boronized material BD 3 (DIN

25 No. 1.2080) according to

Boronizing of the tool made from the same base material resulted in an appreciable cost reduction. This measure was adopted, as the high compressive load necessitated a correspondingly high core strength. Boronizing is carried out for 4 hours in EKabor 2 at a temperature of 850°C (1 123 K). The previously boronized tool is hardened in a neutral (cyanide free) salt bath in accordance with the normal heat treatment for 2.1% C, 12% Cr tool steel. The extremely slight tendency to cold welding of the boride layer is particularly significant in this application. With two strokes per sizing operation, the life of the tool was increased to 800 parts.

The three cone bits of a newly developed rock drill z7 rotate on boronized journals. Roller bearings were used previously. The journals, offset by 120 ° , support the main part of the weight of the material cut during drilling. They are made from SAE EX 30 Cr Ni steel, which is carburized prior to boronizing. The carburizing produces a carbon barrier in the surface layer, which ensures a homogeneous transition of hardness from the boride layer to the base material.

Subsequent to carburizing, the journals are boronized in EKabor for 10 hours at 900°C to 920°C (1 173


to 1 193 K) with subsequent heat treatment to increase the core strength.

The cones, in which the boronized journals rotate, are coated with a silver-manganese alloy, which has an erosive action during rotation. Minute particles of a solid lubricant are released thereby and reach the bearing surfaces between cone and journal. According to the manufacturer, boronizing of the journals increases the load capacity by 15% and/or enables the speed of rotation to be increased.

A dismantled cone bit and journal after 111 hours of operation can be seen in Fig. 24. A depth of about 366 m had been drilled through sand and slate in this time. The drilling force in this case was 200 kN and the speed of rotation of the rock drill was 46 rpm. The operation of this rock drill is extremely economical, as it can remain three times as long in the bore hole as previously and the time to replace the cone bits is short- ened, which means an appreciable cost saving for the contractor.

H. Kunst 2 s has reported on further industrial applications of boronized parts.

Fig. 24 Head section and cone from 9 7/8 in. J22 bit which drilled 1,201 ft of sand and shale in 111 hr at 45,000 lb weight and 46 rpm. Cone inlays were GTAW applied. Bearing pin was pack- boronized (Hughes Tool Comp.)

Short time wear test Short time wear tests on boronized Material specimens have been reported in the No. technical literature; in these tests the parts were subjected both to adhesive

F : 981 N

1.7225 R o t l e ~ s : 100 m

R,ng ~ ~ n: 1~ U/m*. 1.2842

1.2550 1.6580 1.2344 1.2601

Fig. 25 Principle of the friction wear 1.4571 scales according to Reichert

and to abrasive wear s , 2 9 , 3 ° ' 3 4 The apparatus for testing adhesive

wear according to Reichert 31 which has proved convenient in the past for the qualification test of cold roiling emulsion used in the manufacture of strip steel 32, was employed for testing iron boride zones. The working principle of this instrument is described in Fig. 25.

Earlier results/effect of lubricants Rollers and rings of 100 Cr 6 steel

were tested under the conditions indicated in Fig. 25, already reported in t 7. To enable a better comparison with other test methods, for instance with "pin on disk", the lubricant was recently replaced by another type.

In the beginning we used a HD lubricating oil SAE 10W/50, but now a high-efficiency grade with no additive is used, namely BP "Transcal 65", with which a more precise evaluation of the various tests is possible 33

The non-boronized parts had been heat treated by the producer to reach a core hardness of about 61-62 HRC, the boronized parts were not heat treated subsequently. The results are shown in graph 26, from left to right:

Sets of mating parts set 1 : both parts condition as

supplied set 2: ring boronized, roller

condition as supplied set 3: both parts boronized set 4: roller boronized, ring

condition as supplied As can be seen clearly from the

shaded portion of the drawing, the friction surface of sets 1 to 4 decreases, whereas the specific pressure strongly increases in a reciprocal manner (see white bars).

Table 3 Rollers for Re icha r t Tes t

4O

r~ 2

~o

~Ol

i

2

!

3 4

t~00

bor

3000

2

1000

- - 0

Fig. 26 Wearing surface and surface pressure in Reichert test, steel DIN No. 1.2067, plain lubdeating agent

It is interesting to note that the data obtained earlier in industrial practice, were thus corroborated, namely, that it is sufficient to boronize only one part of the set to reduce adhesive wear.

Moreover, this testing method shows the importance of the choice of the parts to be boronized. The abraded area remained smallest (medium value = 0.49 sq. mm) when only the part which was in constant contact and thus exposed to highest duty was boronized, in this case the roller, whereas the ring remained unboronized (set 4). In the present application the specific pressure increased to values of more than 4000 bar. When both parts were boronized, which was unnecessary (set 3), similar values were obtained (medium value: abraded area = 0.63 sq nun/specific pressure about 3100 bar). The boronized ring produced an abraded area of 3.31 sq mm on the non-boronized roller (set 2), which is equal to a specific pressure of about

T y p e (DIN 17006)

2~ Al loy Boron ized Bor ide Weigh t in ~EKabor Layers -% h / " C Fe2B FeB

/an

42 Cr Mo 4

90 Mn Cr V 8

60 W Cr V 7 30 Ni Cr Mo 8 X 4 0 C r M o V 5 1 X 1 6 5 C r M o V 12

6 / 850 70 10 3,2 3 / 950 110 -

3 / 850 40 15 3,6 5 / 950 110 --

4,7 3 / 950 90 - 5,4 3 / 850 35 15 9,5 3 / 900 10 20

15,2 3 / 820 7 7 6 / 900 10 20 X 1 0 C r N i M o T i 18 10 35,4


600 bar; in comparison set 1, with both parts left unboronized, showed an abrased area of 34.0 sq mm or about 60 bar (Fig. 27).

Fig. 27 Rollers and rings Reichert type appar. SAE 52 100 st/BP Transcai 65. After wear test

Results obtained with rollers made from different steels

To examine the extent and the manner which the alloying elements in the matrix, zone thickness and surface condition affect the Reichert test results, rollers of seven different types of steel, were made to the original dimensions (table 3), these ranged from tempering steel to chemically stable steel, types which have already been boronized successfully.

The total amount of alloying elements (~; Leg) was between 3.2 and 35.4 wt%.

As table 3 shows the boronizing conditions chosen were such as to produce duplex boride zones even on some of the steel with a content of alloying element of between 3 and 6 wt%, which is evidence as to the action of FeB.

From Fig. 28 the size of the abraded area determined with the Reichart test may be ascertained. For compar- ative purposes the abraded area o f the 100 Cr 6 (mat. no. 1 2 0 6 7 ) s t e e l rollers (34.0 sq mm) as supplied by the manufacturer and subsequently boronized are shown on the left; that of the non-boronized 1 4571 steel (50.3 sq mm) is shown on the right.

50 3

PhOSl~ZOh[ ~ 2 1 Z 2 2 ? We~kst Nr 2067 7775 ~8~2 .~550 65B0 ~]~z ,S01 6571

Fig. 28 Wear surfaces in Reichert test. Plain lubricating agent

The non-boronized rollers show big abraded areas as compared with those of the boronized rollers, where results obtained range from 0.4 to 0.64 sq. mm. Yet no casual correlation is revealed between abraded area and thickness o f the zone, number o f phases and contents of alloying elements with regard to the present test conditions.

Similar results are obtained in trying to establish a relation between abraded areas and Knoop hardness HK 0.025, and abraded areas and Rt or R a values.

In contrast Habig, Chatterjee- Fischer and Hoffman a4 determined, through the "pin on disk" method, and using a lubricant, a perceptibly lower abrasion value on boronized X 38 Cr Mo V 5 1 steel than on boronized 42 Cr Mo 4 and C 45 steel.

Recapitulating it may be stated that with the above mentioned crucial conditions of the Reichert test the wear behaviour of either non-boronized or boronized rollers differs considerably. On the other hand, potential effects of zone thickness, number of phases, content of alloying elements, micro-hardness and peak-to-valley height are in fact non traceable; this is probably due to the much more stringent test conditions of the Reichert test regarding normal pressure force and rubbing speed and, as a consequence and in proportion, also regarding thermal load.

Hence, the manner and the extent to which the Reichert test conditions may be modified to allow such effects to be detected, require clarification.

References

1. Bundesministerium f//r Forschung u. Technologic, Deutschland (BMFT), Forschungsbericht T 76-38. "Tribologie (Reibung - Verschlei/~-Schmierung)", Juli 1976.

2. Fichtl, W. Harterei-Techn. Mitt., 33 13-20 (1978).

3. DIN 17 014, Marz 1975, Blatt 1 "W~mebehandlung yon Eisenwerk- stoffen".

4. Hansen, M. Constitution of binary alloys, 2. Ed. McGraw-Hill-Book Co. New York, 1958, 251.

5. Fichtl, W. mrv-Metallpraxis 11,431-436 (1972).

6. Fichtl, W. Ind. Anzeiger (88) 2029- 2033, (1973).

7. Fichtl, W. Matusehka, A.v: Oberflache 13,226-232, (1973).

8. Fiehtl, W. Harterei-Teehn. Mitt. 29, (2) 113-119 (1974).

9. ESK-Druekschrift "Hinweisse f//r die Verwendung des ESK-Borierpulvers".

10. Kunst. H. Schaaber, O. H~terei-Techn. Mitt. 22 1-25, (1967).

11. Fiehtl, W. Private Communication. 12. Ct'.atterjee-Fischer, R. Masch-Markt,

33,769-771, (1977). 13. Elektroschmelzwerk Kempten GmbH

"Hinweise fti} die Handhabung der EKabor-Paste".

14. DEGUSSA "Vet fahrensrichtlinien: Borieren mit EKabor-Produekten".

15. Chatterjee-Fiseher, R. Powder Metal- lurgy (2) 96-99, (1977).

16. Dautzenberg, N. Das Borieren yon Sinterstahlen, Vortr'ag anla~l. 5. Europ. Symposium "Pulvermetallurgie" (PM- 78 SEMP), Stockholm, 4. - 8.6.78.

17. Fichtl, W, Berichte Deutsche Kera- mische Gesellschaft 53 156-157 (1976).

18. Badura, H. private communication of 19.10.1977.

19. Fichtl, W. Osterr. Ingenieur-Zeitschrift 22 (11), 426-433, (1979).

20. Just, E. Private Communication of 31.08.1977.

21. VAC-HYD GmbH, D-2358 Kalten- kitchen und Dre-S-Werk, D-8540 Schwabach/Nuiirnberg.

22. Kishimoto, H. Private Communication 17.01.1975.

23. Joost, H.G. "Umformtechnisches Kolloquium" TU Hannover, 17./ 18.03.1977. p. 99-104.

24. Gorelik, S.I. and Others, Kuzn.- Stamp. Proizvod. 18 15-21 (1976).

25. Z~llner, H. MetaUoberflffche 32 455- 458, (1978).

26. Wacker-Chemie GmbH, D-8263 Burghausen.

27. Dill, H.C. Scales, S.R. The Oil and Gas Journal 62-66, (1977).

28. Kunst, H. Durferrit-Hausmitteilungen, (42), 19-22, (1976).

29. Habig, K.H., Kunst, H. H~/rterei- Techn. Mitt. 30 99-107, (1975).

30. St;ihli, G. Beutler, H. Techn. Rundschau Sulzer 1, 1-8, (1976).

31. Reichert, H. Druckschrift "Rebver- sehlei~waage nach Reichert" (DBGM 1749247).

32. Billigmann, J. Fichtl, W. Stahleisen 78 344-357. (1958].

33. Fichtl, W. Practical Applications of the Boronizing Process. Paper in 3. ASM Heat Treating Conference, Detroit, 25. Mai 1977.

34. Habig, K.H. Chatterjee-Fischer, R., Hoffmann, F., in Harterei - Techn. Mitt. 33 28-35, (1978).


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