UDC 621.78:62.762

LOCAL HEAT TREATMENT OF SEALS FROM BERYLLIUM BRONZE BrB2

E. M. Matveev,1 B. M. Gromyko,1 Yu. V. Mityukov,1 I. A. Mikhalev,1 and R. I. Petrenko1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 30 – 32, June, 2003.

The process of local softening of functional surfaces of an elastic seal from bronze BrB2 is studied with theaim of creating a soft layer on the tightening surface, which should perform tightening and protective func-tions. The softening is provided by electron beam treatment (EBT) of the surface layer. The equipment for theEBT is chosen; the properties and the microstructure of the bronze are studied as well as the variation of themicrostructure over the thickness and the width of the softening layer. The tightness of seals serving in simula-tors of detachable joints is determined.

INTRODUCTION

Cryogenic and high-temperature detachable joints (DJ)of fuel conduits and units of liquid-propellant engines (LPE)are equipped with metallic seals with soft tightening coat-ings. The microhardness of coatings from copper and silveramounts to about 130 HV and about 90 HV, respectively. Inthe process of galvanic deposition of copper and silver thematrix material of the seals is charged with hydrogen, sincethe seals of an engine mounted in a carrier rocket (or stored)experience high loads. Free hydrogen localizes in the metalover defects of the crystal structure, which causes itsembrittlement and markedly shortens the life of the seals.This makes the deposition of a galvanic coating an undesir-able operation including the coating of seals fabricated fromberyllium bronze BrB2. We studied the possibility of locallowering of the microhardness only on the regions of func-tional seal surfaces, which contact the matching flange sur-faces, without worsening the high mechanical properties ofthe elastic elements (springs) of the seal.

Analyzing published data [1, 2] we established that themicrohardness of the surface of bronze could be lowered bytwo methods, namely, by quenching or by depleting the sur-face of beryllium.

In order to lower the hardness on local regions of an elas-tic seal produced from quenched and aged bronze BrB2 weneed a lumped source of energy and sufficiently high coolingof the heated regions. When performing local heating of re-gions for decreasing the microhardness by quenching weshould take into account that the minimum heating tempera-ture that provides a single-phase state (�-solid solution) in

bronze BrB2 is about 800°C; at 860 – 870°C there appears aliquid phase, i.e., the temperature range of the heating shouldbe strictly controlled. Quenching at a rate of 30 – 60 K�secyields in BrB2 a supersaturated solid solution, which pro-vides a low hardness.

In order to lower the microhardness of the tightening sur-face by reducing the content of beryllium to 1.5%, when itshardening action is quite low, a local region of the tighteningsurface should be melted with partial evaporation of beryl-lium. The main factor in this case is the tension of the steam,which is much higher than that of copper at the temperatureof beryllium boiling [3].

The aim of the present work consisted in studying the ef-fect of electron beam treatment on the mechanical propertiesand structure of quenched and aged beryllium bronze BrB2and determining the optimum modes of local softening of thefunctional surfaces of the springs of seals produced from thebronze.

METHODS OF STUDY

We used an electron beam installation for size treatmentand welding (ELURS) that made it possible to treat partswith maximum sizes of 300 � 400 � 250 mm at a speed ofdisplacement of the coordinate table from 0.5 to 5 mm�secover the X- and Y-axes at a rotational frequency of 1 – 10 re-volutions per second. The volume of the vacuum chamberwas 0.6 m3; the working vacuum was 8 � 10 – 4 Torr; the timeof creation of the working vacuum was 15 – 20 min.

The electron beam size treatment employs the lumpedenergy freed during the deceleration of the flow of fast elec-trons on the treated region of the surface, in our case on thefunctional surfaces of a seal.

Metal Science and Heat Treatment Vol. 45, Nos. 5 – 6, 2003

2290026-0673/03/0506-0229$25.00 © 2003 Plenum Publishing Corporation

1 V. P. Glushko Énergomash Research and Production Association,Khimki, Moscow Region, Russia.

The treatment was performed so as to provide loweringof the microhardness of the functional tightening surfaces ofan elastic seal (Fig. 1) with a width of about 1 mm and adepth (with allowance for the transition zone) of about0.5 mm; the cross section of the localized zone with loweredmicrohardness must have a lenticular shape. We studied theeffect of different modes of EMT, i.e., the current of the elec-tron beam, the focusing current, the rotational frequency ofthe device, and the diameters of the active zone of the beam,on the microhardness and the structure of the bronze. Wecompared the structure and the microhardness of bronzeBrB2 due to EBT with and without remelting and determinedthe optimum number of passes of the electron beam over thetreated tightening surface. We also performed comparativetests of the tightness of seals before and after EBT.2

RESULTS AND DISCUSSION

We began the study with the diameter of the active zoneof the electron beam equal to 0.15 mm and varied the focus-ing beam current. Analyzing the microhardness we estab-

lished that the zone of lowered microhardness was inconsi-derable, and the values of the microhardness (� 190 HV)were insufficient for providing the requisite tightness of theDJ. In this connection we continued the study for a diameterof the active zone of the beam equal to 0.6 mm. We variedthe cathode current, the focusing current, and the number ofpasses of the beam over the tightening surface, and tried vari-ants with and without melting of the surface.

At one pass of the beam without melting the surface themicrohardness decreased to 188 HV. In the case of melting ofthe surface, the microhardness was 159 HV at a depth of0.05 mm, 156 HV at a depth of 0.15 mm, and 119 HV at adepth of 0.25 HV. The best results were obtained in the treat-ment without melting of the surface (i.e., at a lower cathodecurrent) with repeated passes of the beam (up to six) over thetightening surface; in the latter case the microhardness was119 HV at a depth of 0.05 mm, 152 HV at a depth of 0.15 mm,and 199 HV at a depth of 0.25 mm (Table 1). In the initialcondition the seals had a microhardness of 300 – 340 HV

over the entire cross section.We measured the microhardness along the spring at a step

of 0.5 mm on two seals after five and six passes of the beamover the tightening surface; it amounted to 310 – 340 HV.

The initial microhardness of bronze BrB2 after the heattreatment (quenching + aging, microhardness 300 – 340 HV )is presented in Fig. 2a. It can be seen from the figure that thedistribution of beryllium in the copper grains is not uniform.It seems that beryllium, like hydrogen, has a tendency toconcentrate over grain boundaries of the �-solid solution.

The microstructure of bronze BrB2 in the zone of moltensurface presented in Fig. 2b shows that the melting of thesurface and the subsequent rapid cooling causes formation ofa hardened dendritic structure in the surface layer.

Analyzing the microstructure of the bronze withoutremelting of the surface (Fig. 2c) we established that the de-gree and depth of softening were determined by the mode ofthe EBT, i.e., the specific power of the beam and the time ofthe hold of the bronze at a temperature above 700°C. Thecritical rate of cooling of the surface layers by the beam insuch a case should be at least 60 K�sec due to the removal ofheat to the unheated regions at the expense of the thermalconductivity of the bronze.

230 E. M. Matveev et al.

SZ

Fig. 1. Elastic seal from bronze BrB2 with local softening of thefunctional tightening surfaces (SZ is the softened zone).

TABLE 1. Microhardness of bronze BrB2 after EBT

Numberof beam

passes overthe surface

Microhardness (HV ) at a distance from the surface (mm)

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

1 188 263 300 320 320 300 333 – – – – –

3 119 148 230 263 263 280 300 290 300 275 320 –

4 134 152 178 230 280 321 310 320 – – – –

5 119 135 199 254 270 280 270 265 300 320 – –

6 119 125 152 159 199 230 230 290 300 330 320 –

Melting 159 159 156 144 119 132 112 134 141 156 168 193

2 The study was performed at the Progress NITI (Izhevsk) accord-ing to performance specification of the Énergomash RPA.

Figure 2d presents the microstructure of the tighteningbelt of an elastic seal from bronze BrB2 after six passes of anelectron beam by an optimum mode, on which we can see theplaces of measurement of the microhardness and the varia-tion of the structure over the depth and width of the belt.

The presented results show that in the case of six passesof the electron beam over the treated functional surface ac-cording to the optimum treatment mode the microhardness ofthe tightening belt is close to the least value of the micro-hardness of the bronze after quenching (119 HV ). In thiscase the microhardness in the cross section having a lenticu-lar shape increases monotonically from the center of the sur-face of the cross section towards its edge and into the depthof the belt. At a distance of 0.5 mm from the surface on bothsides of the latter and into the depth of the cross section themicrohardness attains the initial value (about 330 HV ) at adiameter of the electron beam of 0.6 mm.

A fragment of a cross section of an elastic spring of aseal after electron beam treatment with six passes of thebeam over the tightening surface is presented in Fig. 3 (withthe lines of equal microhardness and the boundaries of thebeam-treated zone). It can be seen that the formed zone ofthe effect of the EBT having a lenticular shape is surroundedon three sides by metal with enhanced microhardness. Themicrohardness behind the zone of the effect of the electronbeam on the sealing collar is equal to the initial value. Themicrohardness on the springs and on the bearing ring of theseal also corresponds to the initial value (about 330 HV ),which indicates preservation of the high mechanical proper-ties of the springs.

Analyzing the results of the tests of the tightness of DJ(Table 2) we established that the elastic seals subjected toEBT provide a much higher degree of tightness, especially ata high pressure of the control gas (over 10 MPa), when theeffect of self-compaction is the highest.

The tests of DJ with elastic seals from BrB2 subjected toEBT showed that at a pressure of up to 30 MPa after deposi-tion of a dosed layer of oxygen-resistant SK2-06 lubricationthe tightness is very high (the leakage of helium is below1 � 10 – 5 (liter � �m Hg)�sec]. The seals not treated by elec-tron beam did not provide this degree of tightness at a pres-

Local Heat Treatment of Seals from Beryllium Bronze BrB2 231

330 HV

240 HV

185 HV

140 HV

120 HV

0.05

0.35

0.5

Fig. 3. Fragment of the cross section of an elastic spring with soft-ened zone after EBT.

a

c

b

d

Fig. 2. Microstructure of BrB2 bronze: a) after hardening and aging, � 600; b) in the zone ofmolten surface, � 600; c) in the hardened zone after EBT, � 600; d) tightening surface after sixpasses of electron beam, � 200.

sure exceeding 8 MPa. The width of the contact between thefunctional belts of the seal and the flange surfaces after theEBT ranged within 0.18 – 0.25 mm, which was 30 – 40%greater than the width of the belts on the seals not subjectedto EBT.

CONCLUSIONS

1. We have chosen equipment that provides local treat-ment of functional tightening surfaces and the optimummodes for electron beam treatment of surface layers of tight-ening belts of elastic seals from bronze BrB2, which providelocal lowering of the microhardness to the requisite value of120 HV without sacrificing the initial mechanical propertiesof the springs.

2. Tests of detachable joints with elastic seals frombronze BrB2 have shown that seals with local softening ofthe tightening surfaces by electron beam treatment provide aconsiderable degree of tightness.

REFERENCES

1. A. P. Smiryagin, Commercial Nonferrous Alloys [in Russian],Metallurgizdat (1956).

2. M. V. Mal’tsev, Metallography of Commercial Nonferrous Me-

tals and Alloys [in Russian], Metallurgiya, Moscow (1970).3. K. J. Smithles, Metals, A Handbook [Russian translation], Metal-

lurgiya, Moscow (1980).

232 E. M. Matveev et al.

TABLE 2. Results of Tightness Tests of Seals

Pressureof the controlgas (helium),

MPa

Degree of tightness of DJ,(liter � �m Hg)�sec

sealwithout EBT

seal with EBTafter sizing

1.0 5.0 0.6

5.0 53.0 6.0

10.0 > 140 8.0

20.0 > 140 0.5

30.0 > 140 0.1

20.0 > 140 0.1

10.0 > 140 0.1

5.0 65.0 0.8

1.0 20.0 0.2