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Peculiarities of bimetal material quality estimation.

Bimetals are widely used in various fields of engineering. Although production of various types of bimetals is mastered long ago, there are no common methods for estimating their service properties and metallurgical qualities. Authors of this article made an attempt to find on the basis of analysis of works of various authors, requirements of standards, and own experience generalizing approaches, which would simplify estimation of quality of various bimetals and at the same time increase their authenticity.

Greater part of produced in this country and in the world bimetal materials is represented by hotrolled double-layer corrosion-resistant sheets. According to GOST 10885-85, produced double-layer sheets have thickness from 4 to 60 mm, whereby the base layer is made of carbon or low-alloy steel and the cladding layer consists of corrosion-resistant steels and alloys, nickel, and Monel metal. According to GOST 10885-85 (changes 1 of 01.07.89), on agreement between a manufacturer and a customer double-layer sheets of 120 mm thickness may be produced [1].

Thickness of the corrosion-resistant layer depends upon thickness of a bimetal sheet and should correspond to that indicated in Table 1. It is checked, according to GOST 10885-85, on two specimens, one of which is taken from middle of the sheet cross-sectional template, and the other near the edge. One side of a specimen is ground over its thickness. Thickness of a cladding layer is measured using a magnifying lens or a microscope with error 0.1 mm [2].

Macrostructure of the base layer steel should not have visible (without using magnifying instruments) laminations, accumulations of expanded blisters (separate blisters of maximum 15 mm length are allowed), and soiling. In fractures laminations over the base layer are possible, if their general length does not exceed 20 mm. On requirement of a customer, in the places of double-layer steel bending laminations and cracks should not occur when strength of layer joining and ductility of the base layer are tested.

Fracture tests for determining content of fiber and bend tests of wide specimens are performed according to GOST 5521-76.

Sheets with the base layer made of steel grades 09G2, 09G2S, and 10KhSND on requirement of a customer should correspond to conditions of GOST 5521-76 in bend tests of wide specimens.

According to GOST 10885-85, for determining properties of bimetal steel two test sheets are selected from each lot. Manufactured double-layer sheets are heat treated. Kind and conditions of heat treatment are established by the manufacturer. From each test sheet a strip of 100-200 mm width is cut out, from which specimens are made (1 for tensile strength; 3 for impact bend test for each temperature; 3 for impact bend after mechanical ageing; 2 for bend test, and 3 if thickness exceeds 30 mm; 4 for testing ductility of corrosion-resistant layer; 1 for bend test of wide specimens; 1 for fracture test; 1 for shear test; 1 for checking thickness of corrosion-resistant layer; 1 for checking microstructure; for intercrystalline corrosion test (ICC) number of specimens is established according to GOST 6032-84 from each melting of corrosion-resistant layer in a lot).

[FIGURE 1 OMITTED]

Tensile test is carried out according to GOST 1497-73 on specimens with cladding layer or without it. Sheets of up to 15 mm thickness are tested on flat specimens; of 16-25 mm thickness--on flat and cylindrical specimens; above 25 mm--on cylindrical specimens. Mechanical properties of double-layer sheets should correspond to requirements of standards on a steel grade of the base layer.

Impact bend test of the base layer is performed at normal and reduced temperatures according to GOST 9454-78 on specimens with preliminary removed cladding layer, and after mechanical ageing--according to GOST 7268-82.

In the process of manufacturing equipment for petrochemical industry double-layer corrosion-resistant steel is subjected to various kinds of treatment (bending, forge-rolling, forming, drawing, etc.). That's why it should have sufficient ductility in cold state and be characterized by strong bond, which would ensure absence of the bimetal lamination in the process of technological operations.

Ductility of double-layer sheet steel is checked by cold bend tests carried out according to GOST 14019-80. Mandrel diameter should equal two, and for steel 10Kh2M1--three thicknesses of a specimen. For sheets, having thickness above 30 mm, specimens for bend tests are brought to necessary thickness by machining, whereby one specimen is tested on the side of the cladding layer (for determining ductility of the base layer); two specimens on both sides at the depth proportional to the thickness of each layer (for determining ductility of the cladding layer). Ductility is determined with a tested layer of specimens bent outside.

Strength of joining of layers is determined with cladding layer of specimens bent inside and outside (Figure 1). In case of insufficiently high strength of bond (welding) of the layers, lamination takes place in the place of bend. However, bend tests allow obtaining only qualitative characteristic. Separation of the cladding layer from the base one usually takes place not at once over the whole length of a specimen, and it is difficult to determine the time, at which bond between layers in the bimetal is lost.

One of the most important characteristics of double-layer corrosion-resistant steel is resistance of the cladding layer metal against ICC. Manufacturing of specimens, preparation and performance of tests, estimation of propensity to ICC of the cladding layer from steel of all grades and alloy of the grade 06KhN28MDT are carried out according to GOST 6032-84, and from alloys of grades KhN65MV, KhN65MVU, and N70MFV-VI--according to GOST 24982-81.

For testing resistance against ICC specimens of 20 mm width and 80 mm length are manufactured from the cladding layer metal. Steel of the base layer as well as the boundary zone should be obligatory removed. After boiling specimens in 10 % solution of copper vitriol and sulfuric acid, specimens are bent at 90[degrees] in vice with radius of rounding of jaws or a mandrel not more than three-fold thickness of a specimen (but maximum 10 mm), whereby the side of a specimen adjacent to the removed base layer should be directed inside after bending. Quality of the bent specimen surface is estimated using a magnifying lens of 8--and 10-magnification power. Formation of cross cracks after bending of specimens and loss of the metal sound prove propensity of the cladding layer metal to ICC.

Continuity and guaranteed strength of bond of the base and the cladding layers in a double-layer sheet are characteristics peculiar only to bimetals as structural materials. In case of the bond disturbance each layer works independently and structural properties of the bimetal change. Continuity of bonding of layers, i.e. absence of laminations in the double-layer sheet steel, is checked by the ultrasonic test method according to GOST 22727-88. Check of bond continuity of the double-layer steel of 4-7 mm thickness and steel produced without ultrasonic test is performed on each sheet. Continuity of bonding of sheets of 8 mm and higher thickness has to correspond to requirements presented in Table 2 (GOST 10855-85, change 1 of 01.07.89).

Joining of layers in the bimetal is quantitatively characterized by bonding strength along boundary plane of joining of the base and the cladding layers and perpendicular to it. According to GOST 10855-85, quantitative estimation of the layer joining strength of double-layer sheets is performed by shearing test with determination of shear strength over the contact plane of the base and corrosion-resistant layers. Shear strength, when determining on requirement of a customer strength of joining steel layers, having cladding coating 2 mm and more, should be at least 147 N/[mm.sup.2] (15 kgf/[mm.sup.2]).

Schemes of a specimen and a shearing test of double-layer sheets in correspondence with compulsory supplement to GOST 10885-85 are given in Figure 2. Application of other schemes of specimens is allowed, provided their width and size b are preserved (see Figure 2). Specimens are selected equal to the sheet thickness, allowing for machining on the base layer side. Machining of sheets above 50 mm thickness is performed on the side of the base layer till the thickness achieves 50 mm. When manufacturing specimens, it is necessary to preserve parallelism of machined surfaces for them to be able freely move without jamming in parallel guides, and in case of pressure applied from above downwards shear of the site to take place simultaneously over the whole cross-section.

For estimating quality of bimetal materials other specimens are also used in shearing tests (Figure 3). Special tear tests are envisaged for testing bond (welding) strength by applying load across the plane of joining of layers in bimetal [3, 4]. Schemes of typical specimens and tear tests of bimetal are given in Figure 4.

[FIGURE 2 OMITTED]

Quantitative estimation of bonding strength of layers in bimetal may be performed not only by mechanical separation of bimetal components over the contact surface. For quantitative estimation of quality of bond of layers in auto-vacuum pressure welding, methodology is proposed in [5] based on correlation of the degree of contamination by non-metal inclusions of the boundary zone in laminated metal with data on standard tear tests of these layers. When using such methodology for specific laminated steel, the degree of the layer joint zone purity in regard to the content of non-metallic inclusions has to be compared just ones with tear strength of joined layers. Manufacturing and testing of specimens for tear is not needed.

The main requirement to bimetal rods and rolled round bimetal billets, except guaranteed strength of the core and the cladding layer joining, is production of assigned size of the core of round or close to round form.

In ship-building industry round double-layer rods with the core from high-strength steel and cladding layer from austenite stainless steel are used for manufacturing propeller shafts of high speed ships [6]. As far as it is impossible to get strictly round form of the core in cross section of a rolled double-layer rod, its size in circle may be characterized by diameter of a circumference described around the bimetal rod core, the biggest size of which is determined by assigned diameter of a shaft and minimum allowable thickness of the cladding layer, and diameter of the rod core circumference dr, the smallest value of which is calculated proceeding from the necessary strength. Circumference diameter is calculated by formula [d.sub.r] = = 1.128[square root of (s)], where s is the core section area (it is determined by planimetry of area or some other method).

[FIGURE 3 OMITTED]

In machine-building industry bimetal rods of 20-30 mm diameter with wear-resistant cladding layer from steel Kh12 or 200Kh4F of 3.5-5.0 mm thickness are used for manufacturing caterpillar pins of powerful tractors [7]. It is established that service life of bimetal pins is 7-10 times longer than that of the pins made of steel 50. Application of bimetal rods, manufactured from the compound high-speed steel R6M5 + carbon steel, for example for manufacturing taps, allows increasing their wear resistance 2-3 times and at the same time saving steel R6M5.

In contrast to rolling, deformation in hot extrusion of round bimetal profiles is axisymmetrical, due to which disturbance of the round core form does not occur. That's why hot-pressing dominates till nowadays in production of bimetal rods.

As far as because of the layer surface curvature in hot-extruded bimetal rods application of standard specimens for sheet bimetal is difficult, special specimens were developed for estimating shearing (impactpuncture) and torsion bond strength of layers [8]. Design of a specimen for shearing (impact-puncture) test is given in Figure 5. During puncturing of the core on a test machine shear of a bimetal specimen over bond surface of the components takes place. Shear strength tsh.p may be determined as

[t.sub.sh.p] = [P.sub.p]/[F.sub.sh] [N/[m.sup.2]],

where [P.sub.p] is the core puncturing force, N; [F.sub.sh] is the shear area, [m.sup.2].

[F.sub.sh] = [pi][d.sub.c][t.sub.b],

where [t.sub.b] is the bond zone width, m.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

It should be noted that technical requirements to quality of bimetal rods were developed taking into account technological peculiarities of their production.

So, according to TU 14-3-1222-83 requirements, in bimetal rods manufactured by the method of hot extrusion of hollow centrifugally cast bimetal billets, stipulated by the manufacturing technology transition zone between the cladding layer and the core is allowed, in which number of elements changes within their content in the cladding layer and the core [9]. In measurements of a cladding layer thickness, transition zone is included into the core and its thickness should no exceed 3 mm on each side. In addition, chemical composition of the core metal may have increased content of carbon (up to 0.7 %), chromium (up to 3.0 %), and vanadium (up to 0.3 %) because of partial mixing of metals during pouring of the second layer (the core metal). In connection with this acceptance of bimetal rods is performed according to the results of chemical analysis: for external (cladding) layer--of a ladle sample; for the core (base metal)--of chips taken from the middle of internal layer of transverse macrosection.

Macrostructure of cladding and base layers of a bimetal rod should have dense homogeneous structure. At the same time, over the axis of a bimetal rod, produced by the method of hot-pressing of hollow centrifugally cast bimetal billets, a hole of maximum 6 mm diameter (on edges of the biggest rays) is allowed, inside which non-metallic inclusions may be. It should be noted that this centering hole does not worsen operation properties of bimetal rods [9].

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Transversal macrosections are used for estimating thicknesses of cladding and transition layers and centering hole. Measurements are carried out on toolmaker's microscope. Thickness of cladding layer and allowances are stipulated by respective specifications (for the rods of 25 mm diameter they are [4.sup.+1.0.sub.-0.5] mm).

Strength of the bond between the cladding layer and the core is estimated by the shearing test. It should be at least 180 MPa. The test is carried out on heat treated specimens manufactured on a turning lathe. Cold hardening of the metal should be prevented. Geometrical size of a specimen is selected proceeding from the allowance on external diameter de and cladding layer thickness tcl according to the expression (Figure 6)

[D.sub.1] = [d.sub.e min] - (2[t.sub.cl max] + 2 mm);

[D.sub.2] = [d.sub.e max] - (2[t.sub.cl min] - 2 mm).

Specimens are tested on test machines, which meet requirements of GOST 7855-74 and ensure loading of a specimen by compression force between planeparallel stops at a speed 2 mm/min. In test of specimens maximum force P is determined, which corresponds to the moment of the specimen fracture with error up to 9.8 N. Shear strength of the layer bond is calculated by formula tsh = Psh/[F.sub.sh] [N/[m.sup.2]], [F.sub.sh] = = [pi][D.sub.s]h, where h is the height of cylindrical belt of a shear surface equal to 2.5 mm; [D.sub.s] is the separation diameter. Bond strength is calculated with error up to 1 MPa.

High prospects for expansion of production of bimetal round profile are opened in using method of electroslag cladding with liquid metal (ESC LM) of bimetal billets and their subsequent hot deformation (rolling, pressing, etc.) [10, 11].

Length of transition zone in bimetal corrosion-resistant reinforcement profile of 16 mm diameter, rolled from bimetal billet 20GS +316L of 350 mm diameter clad by ESC LM method, is only 6-12 mm and does not depend upon thickness of the cladding layer [12].

[FIGURE 8 OMITTED]

Quality of the cladding layer and the core bond in bimetal profile of 16 mm diameter was estimated by cold bend test of special specimens. Longitudinal macrosections having thickness equal to half diameter of bimetal profile were used. Bending was performed on a mandrel, diameter of which equaled two diameters of the profile, whereby surface of the macrosection was bent outside (Figure 7). Separations of cladding layer in the specimen after bending was not detected. Study of transition zone microstructure of bimetal profile (Figure 8) also showed high quality of the bimetal.

For wide introduction of bimetal reinforcement with corrosion-resistant cladding layer it is necessary to develop special specifications, which would regulate test of not only a bimetal profile, but also of its welded joints.

[1.] GOST 10885-85: Sheet hot-rolled double-layer corrosion-resistant steel. Specification. Moscow: Standart.

[2.] Piryazev, D.I., Khoroshilov, N.M., Kuzmenko, Yu.A. (1966) Production of bimetal sheet steel. Kiev: UkrNIINTI.

[3.] Chepurko, M.I., Ostrenko, V.Ya., Gluskin, L.Ya. et al. (1984) Bimetal materials. Leningrad: Sudostroenie.

[4.] Potapov, I.N., Lebedev, V.N., Kobelev, A.G. et al. (1986) Laminated metallic combinations. Moscow: Metallurgiya.

[5.] Paton, B.E., Medovar, L.B., Saenko, V.Ya. et al. (1981) Evaluation of quality of layer adhesion in autovacuum pressure welding. Avtomatich. Svarka, 10, 19-21.

[6.] Vejngarten, A.M., Delle, V.A., Noskin, A.V. et al. (1962) Shipbuilding steel. Leningrad: Sudpromgiz.

[7.] Makovsky, V.A., Ejlman, L.S. (1981) Bimetal rods. Moscow: Metallurgiya.

[8.] Chernov, A.N., Golovanenko, S.A., Gulyaev, V.V. (1965) Study of layer adhesion strength in hot-rolled bimetal. In: Transact. on Production of Bimetals. Moscow: Metallurgiya.

[9.] Chepurko, M.I., Ostrenko, V.Ya., Kogadeev, A.A. et al. (1986) Production of bimetal pipes and rods. Moscow: Metallurgiya.

[10.] Paton, B.E., Medovar, L.B., Saenko, V.Ya. (2004) About prospects of bimetal production using electroslag process. Advances in Electrometallurgy, 4, 2-6.

[11.] Medovar, L.B., Saenko, V.Ya., Us, V.I. et al. (2005) Specifics in designing of bimetal billets for producing reinforcement bars with a corrosion-resistant cladding layer of steel 316L. Ibid., 2, 8-12.

[12.] Medovar, L.B., Saenko, V.Ya., Remizov, A.G. et al. (2005) Peculiarities of structure of bimetal reinforcement bar with a corrosion-resistant clad layer of steel 316L. Ibid., 3, 18-21.

V.Ya. SAENKO, L.B. MEDOVAR, B.B. FEDOROVSKY, N.T. SHEVCHENKO, V.M. YAROSH, V.V. ZHUKOV, V.M. ZHURAVEL, V.A. ZAJTSEV, R.V. KOZIN and A.G. REMIZOV

E.O. Paton Electric Welding Institute, NASU, Kiev, Ukraine
Table 1. Requirements to thickness of cladding layer of bimetal sheets

Sheet thickness, mm Thickness of corrosion-resistant layer, mm

 Normal Increased

 4 0.7--1.1
 5 0.8--1.2
 6 1.0--1.6
 7 1.2--1.8 Not envisaged
 8, 9 2.0--3.0
 10--15 2.0--3.0 3--4
 16--21 2.5--3.5 3--4

Sheet thickness, mm Thickness of corrosion-resistant layer, mm

 Normal Increased

 22, 24--26 3--4
 28, 30 3.5--5.0

 32, 34, 36, 3 4--6
42, 45, 48, 50, 52, Not envisaged
 55, 60

Table 2. Requirements to allowable area of bimetal sheet
discontinuities

Class of Conditional area of Conditional area of
 sheets discontinuities, [cm.sup.2] maximum allowable
 zone of discontinuities,
 [m.sup.2]

 Minimum Maximum
 accounted allowable

 01 On agreement
 between
 manufacturer
 and a customer
 0 5 20 1

 1 10 50 2
 2 20 100 2
 3 50 250 --

Class of Relative conditional maximum area of Maximum allowable
 sheets all accounted metal discontinuities, length of discon-
 % not more than tinuities,
 [mm.sup.2]

 Per 1 [m.sup.2] Per unit of rolled
 sheet area

 01 On agreement between manufacturer
 and a customer
 0 1 0.3 30 (for rolled sheet
 of up to 60 mm
 thickness), 50
 (for rolled sheet of
 above 60 mm
 thickness)
 1 2 0.5 50
 2 3 1 100
 3 5 2 200
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Title Annotation:ELECTROSLAG TECHNOLOGY
Author:Saenko, V. Ya.; Medovar, L.B.; Fedorovsky, B.B.; Shevchenko, N.T.; Yarosh, V.M.; Zhukov, V.V.; Zhura
Publication:Advances in Electrometallurgy
Article Type:Report
Date:Jan 1, 2006
Words:3274
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