High Performance Internal Combustion Engine Exhaust Valve Alloys ISO9001

Alloy LF8 (LF8 valve alloy) for high performance internal combustion engine exhaust valve

PRODUCT

Alloy LF8 (LF8 valve alloy) for high performance internal combustion engine (diesel engine and gasoline engine) exhaust valves for automobile, locomotive, tractor, ship, tank, oil rig, construction machinery and mobile power station, etc. Also could be for high strength fasteners at elevated temperatures.

PRODUCT FORM

Bar and rod: the delivery condition is rolled, heat treated, oxidation, descaling, turned, ground, and polished, etc.

Others: disc, seamless pipe and tube, cylinder, forging, forging block etc.

APPLICATION

Alloy LF8 is mainly used in the exhaust valve of high performance internal combustion engine under the working temperature up to 750°C. Because Alloy LF8 has a higher strength and hardness at room temperature and high temperature than Alloy 80A, it is expected to be the preferred material for valve alloy up to high working temperature of 750°C.

SKETCH OF EXHAUST VALVE

PRODUCTION PROCEDURE OF EXHAUST VALVE

Blanking → Electric heating upsetting forging of head blank → Heat treatment of head blank and rod → Friction welding → Rough turning or grinding → Finish turning → Cut fixed length → Semi-fine grinding the stem → The valve stem chrome plating → Fine grinding the stem → NDT of the finished valve → Delivery

SURFACE CONDITION OF EXHAUST VALVE

PRODUCTION SITE OF EXHAUST VALVE

CHEMICAL COMPOSITION (wt%):

Table 1

OVERVIEW

Internal combustion engine exhaust valves works in high temperature gas corrosion and high stress action and other harsh environment, the exhaust valve to withstand the temperature of up to 600-800°C. Alloy 80A and Alloy 751 are two commonly used valve alloys. With the large quantity of application, Alloy 80A gets more and more attention for its high temperature performance. After study of the microstructure and properties of Alloy 80A, it was found that the increase of Ti/Al ratio significantly improved the mechanical properties at room temperature. When Ti/Al is relatively low, β-NiAl phase is precipitated out of the crystal, and will result in high temperature fracture of the material.

As the requirements for emission reduction continue to increase, the requirements for engine efficiency continue to increase, and the combustion chamber temperature is also further improved. According to the current research on the high temperature performance of exhaust valve alloy, it is found that Alloy 80A and Alloy 751 can be used in about 700°C, but when the temperature reaches 750°C, the high temperature performance of this kind of alloy appears insufficient, and often causes the failure of the exhaust valve when working. Therefore, in order to adapt to the rising working environment temperature of exhaust valve, a new type of valve alloy with better performance than Alloy 80A needs to be developed, which works around 750°C.

Alloy LF8 for exhaust valve was developed based on Alloy 80A to study the effect of Cr, Al,Ti and Co on precipitated phase.

The study showed that with the increase of Cr content, γ' phase increased slightly, indicating that Cr had little effect on γ' phase. The increase of Cr content first led to the transformation of carbide type from M₇C₃ to M₂₃C₆, and then the number of M₂₃C₆ increased with the increase of Cr content. When Cr content exceeded 20%, a large number of α-Cr phases appeared in the alloy.

With the increase of Al content,γ' phase significantly increased, carbides M₂₃C₆ slightly increased, indicating that Al was the main forming element of γ' phase, but also participated in the formation of carbides M₂₃C₆.

γ' phase content increased with the increase of Ti content, but when Ti content reaches 4.5%, a large number of η brittleness phases existed in the equilibrium precipitated phase, with a content reaching 10.634%, so the Ti content in the alloy shall range from 3.5-4.0%.

With the increase of Co content, the number of γ' phase and M₂₃C₆ phase was basically unchanged, indicating that Co did not participate in the formation of γ' phase and M₂₃C₆ phase, but only existed in the matrix in the form of solid solution.

The analysis showed that the increase of Cr element content slightly increased the amount of γ' phase, which not only changed the carbide type, but also increased the amount of M₂₃C_6. The element Cr mainly increase the ability of oxidation and corrosion resistance. But excessive Cr content may form α-Cr phase, so the content shall be controlled at 17-20%. The increase of Al and Ti can significantly increase the precipitation of γ' phase and is an important forming element of γ' phase. But although increasing the content of Ti and Al increases the content of γ' phase, to avoid η brittleness phase, the content of Ti+Al should be 5.5-7.0%, and the Ti/Al ratio should be 1.16-2.00. The addition of Co had little effect on γ' phase and M₂₃C₆ phase, but it can strengthen the alloy by solid solution. The element Co can reduce the solubility of Al and Ti elements in γ matrix and play a role of solid solution strengthening, and can be appropriately added to increase the strength of the alloy.

Based on the above studies, Cr content was increased in order to improve the oxidation resistance of the alloy, Fe content was increased in order to reduce the cost of the alloy and the amount of Ni was reduced. The specific composition are shown in table 1 above.

METALLOGRAPHY

Figure 1 SEM micrograph showing microstructure and the corresponding energy spectra of the alloy after heat treatment

Figure 2 TEM micrograph of precipitated phases and diffraction patterns of the alloy

Table 2 Precipitation phase of the alloy after heat treatment

Figure 1 SEM micrographs showing microstructure and the corresponding energy spectra of the alloy after heat treatment

( a) micrograph scan; ( b) grain boundary carbides; ( c) EDS spectrum of M₂₃C₆; ( d) EDS spectrum of MC

Fig 2 TEM micrographs of precipitated phases and diffraction patterns of the alloy

( a) γ'phases; ( b) TiC phase; ( c) M₂₃C₆ phases

Table 2 Precipitation phase of the alloy after heat treatment

It can be seen from figure 1 and figure 2 that the micro-structure of Alloy LF8 after heat treatment is austenitic matrix with a large number of annealing twins. The grain size varies from 20 microns to 150 microns. γ', M₂₃C₆ and TiC phases are precipitated. According to the thermodynamic calculation results, γ' phase is the main strengthening phase in Alloy LF8, which plays the role of precipitation strengthening. When γ' phase grows up, interface energy will be increased to increase the instability of the system. γ' phase precipitates out in the aging process of heat-resistant alloy and is affected by both temperature and time. In Alloy LF8, the γ' phase was very small after 760°C / 5 hours aging. The γ' phase was not distinguishable under scanning electron microscope (SEM) as shown in figure 1. The small γ' phase in the matrix could be clearly seen in figure 2. γ' phase in Alloy LF8 is nearly spherical and distributed in the crystal. The size is about 20nm. Alloy LF8 has a short aging time, and the smaller size and less content of γ' phase were at the initial stage of precipitation without coarsening or growth. Table 2 is the qualitative results of chemical extraction and X-ray diffraction phase analysis of Alloy LF8 after heat treatment. It shown from the table the γ' ɑ lattice constant 0 = 0.357 to 0.358 nm,γ' is dissolved by Cr in the alloy, γ' phase quantity increased slightly with the increase of Cr content. As can be seen from the scanning photos in FIG. 1(b) and the energy spectrum photos in FIG. 1(d), Cr₂₃C₆ is the main precipitated carbide, showing a discontinuous ellipse with a length of 400-800nm. Cr₂₃C₆, which is partially distributed in the crystal, is in a circular spot shape. See from table 5 that lattice constant ɑ 0 = 0.430 to 0.431 nm, Cr and Ni in the alloy were dissolved into M₂₃C₆ to form Cr₂₃C₆. Cr₂₃C₆ distributed at the grain boundary acts as a nail binding relative to the grain boundary and can effectively increase the high-temperature strength of the alloy. Continuously distributed Cr₂₃C₆ phase will reduce the interface energy, but discontinuous distribution of Cr₂₃C₆ has a better effect on grain boundary pinning effect, and the size should not be too large. If the aging time is too long, Cr₂₃C₆ phase is prone to aggregation and growth, which will affect the high temperature performance of the alloy. It can be seen from the scanning photos in FIG. 1(a) and the energy spectrum photos in FIG. 1(c) that the carbides precipitated from the crystal are MC, which are small blocks with a small quantity and a size of 500-1000nm. From the transmission photo (FIG. 2b), TiC, which is in the form of a short bar, can also be clearly observed.Table 2 shows the lattice constant of MC phase ɑ 0 = 1.060 to 1.062 nm, which is relative large. TiC can be divided into primary and secondary forms. Primary TiC carbides are formed in the solidification process and are mostly distributed within and at grain boundaries. The average size of TiC carbides is relatively large.Secondary TiC is precipitated from γ' matrix or transformed by other phases during the cooling and heat treatment of hot processed alloys or long-term use. Primary TiC is relatively stable in hot processing and heat treatment because of its large size and high precipitation and dissolution temperature. From the thermodynamic software, it can be seen that there was no TiC equilibrium phase precipitated in 760°C equilibrium phase. The precipitated phases calculated by thermodynamic software were all equilibrium precipitated phases, excluding undissolved or other transition phases. The TiC existing in the alloy should be a small amount of primary TiC in the part with high solubility that was not dissolved back.

MECHANICAL PROPERTIES

Figure 3 Comparison of tensile properties and hardness of Alloy LF8 and Alloy 80A

Figure 4 Mechanical performance of Alloy LF8 at high temperature of the tested samples after the conventional heat treatment

Figure 5 Equilibrium thermodynamic phase diagram of the alloy

Fig 3 Comparison of tensile properties and hardness of Alloy LF8 and Alloy 80A

Fig 4 Mechanical performance of Alloy LF8 at high temperature of the tested samples after the conventional heat treatment ( a) tensile strength; ( b) yield strength

Fig 5 Equilibrium thermodynamic phase diagram of the alloy (a) Alloy LF8 equilibrium state thermodynamic phase diagram; (b) Alloy 80A alloy equilibrium state thermodynamic phase diagram.

It can be seen from figure 3 that Alloy LF8 has tensile strength of 1307MPa and yield strength of 973MPa respectively, and its hardness is 40.8HRC. Alloy 80A has 1194MPa tensile strength and 776MPa yield strength at room temperature, and its hardness is 37.6HRC. Alloy LF8 is 8.6%, 20% and 7.9 higher than Alloy 80A, respectively.

It can be seen from figure 4(a) 5(b) that the tensile strength and yield strength of Alloy LF8 and Alloy 80A decreased with the increase of temperature. The tensile strength and yield strength of Alloy LF8 at 750°C were 845MPa and 750MPa, while those of Alloy 80A at 750°C were only 802MPa and 657MPa. The tensile strength and yield strength of Alloy LF8 were significantly higher than those of Alloy 80A at 750°C, which were 5.0% and 12.4% higher respectively.

The content, size and distribution of the precipitated phase in the aging state have a great impact on the strength of the metal material, and the stability of the microstructure after aging will also have an impact on the mechanical properties of the alloy. γ' and carbides are important strengthening phases of nickel-based alloys. In nickel-based heat-resistant alloys, there is a co-lattice relationship between γ' and the substrate. After aging, the mismatch between γ' of LI2 structure and the substrate increases, which is easy to be converted into a more stable cubic structure. After 760°C / 5 hours aging, Alloy LF8 was strengthened by precipitation of γ' phase and carbide from grain boundary. Figure 5 is the calculation result of thermo-calc thermodynamic software. According to the equilibrium phase diagram, the precipitated content of Alloy LF8 γ' phase in 760°C equilibrium phase was 27.21%, and Alloy 80A only 18.60%. Alloy LF8 was 8.61% higher than Alloy 80A's γ' equilibrium precipitated phase. This indicated that γ' phase precipitated in Alloy LF8 was greater than that in Alloy 80A at 760°C, so the strength of Alloy LF8 was theoretically higher than that of Alloy 80A. At the same time, Co was added to the alloy to increase the effect of solid solution strengthening and reduce the dissolution of γ' phase. Cracks in the grain boundary at high temperature are often the main reasons for the premature failure of the alloy. Carbon tends to diffuse to the grain boundary at high temperature, so that Cr-rich carbides at the grain boundary accumulate and grow up, and finally form lamellar brittle phase to reduce the high-temperature strength and toughness of the alloy. Compared with nickel-base heat-resistant alloys such as Alloy 80A, Alloy 751 and Alloy 617, grain boundary carbides were discontinuous in Alloy LF8 after heat treatment. The carbide with this morphology can effectively nail the grain boundary, improve the bonding force of the alloy grain boundary, increase the resistance of grain boundary slip, reduce the formation of grain boundary crack source, and improve the resistance of grain boundary to tensile.

The data analysis of mechanical experiments showed that Alloy LF8 had higher strength and hardness than Alloy 80A, and it was expected to be the preferred alloy material for internal combustion engine exhaust valve at working temperature up to 750°C.

COMPETITIVE ADVANTAGE:

(1) More than 50 years experience of research and develop in high temperature alloy, corrosion resistance alloy, precision alloy, refractory alloy, rare metal and precious metal material and products.
(2) 6 state key laboratories and calibration center.
(3) Patent technologies.

(4) Average grain size 9 or finer.

(5) High performance

BUSINESS TERM