HEAT TREATMENT METHOD FOR ACCELERATING PRECIPITATION OF NANOSCALE CARBIDES IN W-CONTAINING ALLOY STEEL

Abstract

A heat treatment method for accelerating precipitation of nanoscale carbides in a W-containing alloy steel, including: homogenizing a billet at 1100-1200 C. for 24-48 h followed by furnace cooling to room temperature; then austenitizing the billet at 850-1000 C. for 20-40 min followed by quenching in iced brine; heating the billet to 650-750 C. with a rate of 3-7 C./min under a vacuum of 10.sup.3-10.sup.2 Pa and a magnetic field of 10-14 T, isothermalizing the billet for 0.5-2.5 h and cooling the billet to room temperature. The chemical composition of the billet is: 0.06-0.14 wt % C, 1.50-3.00 wt % W, <0.01 wt % P, <0.005 wt % S, Fe and an inevitable impurity. A temperature of the iced brine is 3 to 1 C. The invention has the advantages of simple process, low cost and shortened production period. The W-containing alloy steel treated by the method has improved strength.

Claims

1. A heat treatment method for accelerating precipitation of nanoscale carbides in a W-containing alloy steel, comprising: homogenizing a billet at 1100-1200 C. for 24-48 h followed by furnace cooling to a room temperature; austenitizing the billet at 850-1000 C. for 20-40 minutes followed by quenching in iced brine; and heating the billet to 650-750 C. with a rate of 3-7 C./min under a vacuum of 10.sup.3-10.sup.2 Pa and a magnetic field of 10-14 T, isothermalizing the billet for 0.53.0 h and then cooling the billet to room temperature; wherein a chemical composition of the billet is 0.06-0.14 wt % C, 1.50-3.00 wt % W, <0.01 wt % P, <0.005 wt % S, Fe and an inevitable impurity.

2. The method of claim 1, wherein a temperature of the iced brine is 3 to 1 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by a heat treatment method according to the present invention.

[0016] FIG. 2 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by the method in FIG. 1 when no magnetic field is applied.

[0017] FIG. 3 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by another heat treatment method according to the present invention.

[0018] FIG. 4 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by the method in FIG. 3 when no magnetic field is applied.

DETAILED DESCRIPTION OF EMBODIMENTS

[0019] The invention is further described below in conjunction with the drawings and embodiments, which are not intended to limit the scope of the invention.

Example 1

[0020] This embodiment provides a heat treatment method for accelerating the precipitation of nanoscale carbides in a W-containing alloy steel.

[0021] The billet was homogenized at 1100-1150 C. for 36-48 h followed by furnace cooling to room temperature. The billet was austenitized at 900-1000 C. for 20-30 minutes and quenched in iced brine with a temperature of 3 to 1 C. Then, the billet was heated to 700-750 C. with a rate of 3-7 C./min under a vacuum of 10.sup.3 to 10.sup.2 Pa and a magnetic field of 10-13 T. The billet was isothermally held for 0.5-2.5 h, and then was cooled to room temperature.

[0022] The chemical composition of the billet is 0.06-0.12 wt % C, 1.50-2.50 wt % W, <0.01 wt % P, <0.005 wt % S, Fe and inevitable impurities.

[0023] FIG. 1 is a transmission electron micrograph showing precipitation of nanoscale carbides in a W-containing alloy steel by the heat treatment method according to the present invention. FIG. 2 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by the method in FIG. 1 when no magnetic field is applied.

[0024] As shown in FIGS. 1 and 2, the nanoscale particles are M.sub.6C carbides. It can be seen from the comparison between FIGS. 1 and 2 that the amount of nanoscale carbides is significantly increased after the product is subjected to the tempering heat treatment with a 12 T magnetic field. Compared with the heat treatment without magnetic field, the surface density of carbides is increased by about 39%, indicating that the application of high magnetic fields significantly increases the nucleation rate of the carbide, which enhances the precipitation hardening and thus the strength of W-containing alloy steel.

Example 2

[0025] This embodiment provides a heat treatment method for accelerating precipitation of nanoscale carbides in a W-containing alloy steel. First, the billet was homogenized at 1150-1200 C. for 24-36 h followed by furnace cooling to room temperature. Then, the billet was austenitized at 850-950 C. for 30-40 minutes, and was quenched in iced brine with a temperature of 3 to 1 C. Finally, the billet was heated to 650-700 C. with a rate of 3-7 C./min under a vacuum of 10.sup.3-10.sup.2 Pa and a magnetic field of 11-14 T and was isothermally held for 1.0-3.0 h, then was cooled to room temperature.

[0026] The chemical composition of the billet is 0.08-0.14 wt % C, 2.00-3.00 wt % W, <0.01 wt % P, <0.005 wt % S, Fe and inevitable impurities.

[0027] FIG. 3 is a transmission electron micrograph showing precipitation of nanoscale carbides in a W-containing alloy steel by another heat treatment method according to the present invention. FIG. 4 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by the method of FIG. 3 when no magnetic field is applied.

[0028] As shown in FIGS. 3 and 4, the nanoscale particles are M.sub.6C carbides. It can be seen from the comparison between FIGS. 3 and 4 that the amount of nanoscale carbides is significantly increased after the product is subjected to the tempering heat treatment with a 12 T magnetic field. Compared with the heat treatment without magnetic field, the surface density of carbides is increased by about 55%, indicating that the application of strong magnetic fields significantly increases the nucleation rate of the carbide, which enhances the precipitation hardening and thus the strength of W-containing alloy steel.

[0029] The present invention has the following advantages as compared to the prior art.

[0030] 1. In the heat treatment method of the invention, the alloying elements are completely diffused after the billet is homogenized at a high temperature, and the banded structure during hot-rolling is eliminated to avoid a non-uniform structure. The billet is austenitized and water-quenched to obtain a lath martensite followed by a tempering heat treatment in a high magnetic field. Such heat treatments lead to a simple process, shortened production cycle and improved production efficiency.

[0031] 2. In the present invention, only the alloying element W is added which greatly reduces the cost compared with other patents or applications in which complex and expensive alloying elements are employed for the production of dispersed carbides. Further, low-carbon and high-tungsten alloys are used to achieve high-temperature creep strength under the strong magnetic field. Alloy carbides such as M.sub.7C.sub.3, M.sub.23C.sub.6, M.sub.6C are important constituent phases in heat-resistant steels, and their precipitation and evolution have an extremely important influence on the high-temperature creep properties of heat-resistant steels, such as low-activated steels. M.sub.23C.sub.6 carbide tends to be coarsened at a grain boundary under a high-temperature service. Reduced C content can lower the coarsening of M.sub.23C.sub.6 at the grain boundary, and increased W content can stabilize the M.sub.23C.sub.6 at the grain boundary and improve the solid solution strengthening, thus improving the strength of the W-containing alloy steel.

[0032] 3. In the present invention, a high magnetic field is applied during the tempering process of W-containing steel, and a structure having a large amount of dispersed nanoscale particles with ferrite as a matrix is manufactured. The application of the high magnetic field increases the driving force of phase transformation, so that the nucleation barrier of carbide is lowered and the nucleation rate of the carbide is increased. Without the application of magnetic field during tempering, the surface density of the precipitation is about 22.9%, and the carbides' size is 5-15 nm. When the magnetic field is applied, the carbides' size is substantially unchanged, but the surface density of the carbides increases to 31.6%, which is about 39% higher than that of the carbides when no magnetic field is applied. It can be seen that the high magnetic field significantly accelerates the precipitation of nanoscale carbides in the W-containing steel during the tempering process, enhancing the precipitation hardening and thus the strength of W-containing alloy steel.

[0033] Therefore, the heat treatment method according to the present invention is simple and has a low-cost process and a short production cycle, which apparently improves the strength of the W-containing alloy steel.