Method of fabrication of composite material based on vanadium alloy and steel

Abstract

The method of producing composite material with a high complex of mechanical properties, consisting of vanadium alloy inner layer V—3-11 wt % Ti—3-6 wt % Cr and two outer layers of stainless steel of ferritic grade with chromium content of not less than 13 wt %, includes preparation of a composite workpiece consisting of said inner layer and outer layers, hot treatment by pressure and subsequent exposure in furnace. Prepared composite workpiece, thickness of inner layer of which is 1.5-2 times more than total thickness of outer layers of stainless steel, hot working is performed with pressure of the workpiece in the temperature range of 1,050-1,150° C. with degree of reduction from 30 to 40% and with subsequent exposure for 1-3 hours with temperature reduction to 500-700° C., then annealing workpiece by heating to temperature of 850-950° C., holding for 2-4 hours and subsequent cooling in furnace.

Claims

1. Method of fabricating a composite material comprising an inner layer of V-3-11 wt. %Ti-3-6 wt. %Cr vanadium alloy and two outer layers of stainless ferritic steel containing at least 13 wt. % chromium, said method comprising preparation of a composite material billet comprising said inner layer and two outer layers and hot pressure treatment followed by tempering in the furnace wherein said composite material billet is prepared such that the thickness of said inner layer is 1.5-2 times greater than the total thickness of said two outer layers of stainless steel, said composite material billet is hot pressure treated in the 1050-1150° C. range with a reduction of 30-40% followed by tempering for 1-3 h during temperature reduction to 500-700° C., annealed by heating to 850-950° C., tempered for 2-4 h and cooled in the furnace.

2. Method of claim 1 wherein said hot pressure treatment is hot pressing or hot rolling.

3. Method of claim 1 wherein said hot pressure treatment and tempering are effected in a protective atmosphere.

Description

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0010] The herein disclosed method of fabrication of composite material on the basis of vanadium alloy (the vanadium/titanium/chromium system) and stainless steel (chosen from ferrite steels) comprises hot pressure treatment of the composite material billet in a protective atmosphere at temperatures in the 1050-1150° C. range with a 30-40% reduction followed by tempering in the furnace which is implemented as a stepwise process, i.e., comprises cooling from the hot treatment temperature to 500-700° C., tempering for 1-3 h, heating to 850-850° C., tempering for 2-4 h and cooling in the furnace so the overall time of tempering in the furnace reaches 3-7 h.

[0011] The method disclosed herein provides for the formation of a diffusion bonding area between the vanadium alloy and steel with a large thickness of 60-70 μm with an insignificant increase in the grain size of the vanadium alloy and steel, reduction of residual stresses and absence of second phase precipitation, which for the preset ratio of layer thicknesses in the composite material billet provides for an improved set of mechanical properties of the composite material.

[0012] An important aspect of the method disclosed herein is that the increase in the overall heat treatment (annealing) time delivers an increase in the thickness of the diffusion transition area of the bond, a more uniform structure and a reduction of residual stresses over the material cross-section due to recrystallization processes, while avoiding the expected significant increase in the grain size of the composite material components and second phase precipitation at the bonding interface (due to the implementation of a stepwise tempering sequence) and hence delivering an improved set of mechanical properties of the material. Furthermore the method disclosed herein provides for lower power consumption due to the phasing out of additional reheating before annealing.

[0013] Increasing the time of tempering after heat treatment to several hours is acceptable in the practice of heat treatment unless it causes undesirable consequences such as the formation of brittle compounds at the bonding interface or an abrupt growth of grain size in the components of the composite material. The use of slightly lower tempering temperatures (500-700° C.) somewhat decelerates structural evolution processes in the composite material but develops auspicious conditions for diffusion processes which increase the thickness of the diffusion transition area between the components and increases the strength of the bond.

[0014] The method disclosed herein is implemented as follows. The composite material billet is prepared using known conventional methods in the form of a sheet, a tape, a pipe or a rod comprising an inner layer of vanadium alloy (V-3-11 wt. %Ti-3-6 wt. %Cr) and two outer layers of stainless steel (chosen from ferritic steels with a chromium content of at least 13 wt. %). The thickness of the vanadium alloy layer in this composite material billet is 1.5-2.0 times greater than the total thickness of the steel layers. The composite material billet is hot pressed or hot rolled in a protective atmosphere at a temperature in the 1050-1150° C. range with a reduction of 30-40%. Then the pressed billet is cooled down to a temperature in the 500-700° C. range during 1-3 h in the protective atmosphere, then heated to 850-950° C., tempered (annealed) for 2-4 h in the protective atmosphere and finally cooled in the furnace.

[0015] To implement one of the embodiments of the method disclosed herein the instant inventors used by way of example a three-layered sheet billet of V-4%Ti-4%Cr alloy with a thickness of 1850 pm located between two layers of 08Cr17Ti stainless steel which were located under the bottom and on the top of the vanadium alloy layer and had a total thickness of 300 μm. The three-layered billet was prepared in a conventional way including surface machining and vacuum treatment. The composite material billet was hot rolled in a protective atmosphere at 1100° C. The thickness of the as-hot rolled three-layered billet was 1750 μm. After hot rolling the three-layered billet was cooled down to 600° C. for 2 h in the protective atmosphere. Then the billet was transferred to the furnace and annealed at 900° C. for 3 h in the protective atmosphere of argon gas and cooled down in the furnace.

[0016] After the treatment, the billet was cut into specimens in different areas of billet length for materials science study (analysis of microstructure and chemical element redistribution in the bonding area). The results of analysis showed that the thickness of the diffusion transition area of the bond was 70±5 μm, no second phase precipitation occurred at the bonding interface layer and the steel grain size in the vicinity of the bonding interface was 65±5 μm. The bonding interface did not contain any defects (cracks, exfoliation etc.). Tensile tests of the bimetallic microscopic specimens cut perpendicular to the pipe walls showed a good set of mechanical properties (σ.sub.0.2310±12 MPa, σ.sub.B=450±15 MPa and δ=20±2%) and their better reproducibility over the pipe length (the mechanical parameters were reproducible accurate to ±5-7% along the pipe). Thus the tests showed that the use of the method disclosed herein allows achieving a significant increase in the thickness of the diffusion transition area without second phase precipitation or significant grain size growth of composite material components at the bonding interface. This provides for an improved set of mechanical properties of the composite material and stable mechanical properties in the pipe length.