JP5727903B2 - Co-base alloy for surface hardening - Google Patents

Description

  The present invention relates to a surface-hardening Co-base alloy having wear resistance and heat resistance and high toughness applied by thermal spraying or overlay welding, and in particular, this surface-hardening Co-base alloy. Since it has age-hardening properties, it is characterized by increased hardness and further improved wear resistance when exposed to a high temperature environment for a long time. Further, since it has high toughness, it is characterized by being resistant to thermal shock and adaptable to surface coating on a member having a large coefficient of thermal expansion such as austenitic stainless steel.

  Conventionally, surface-hardening alloys used in high-temperature environments include nickel or cobalt self-fluxing alloys containing high Cr as defined in JIS H 8303: 2010, and Co-Cr-WC alloys represented by stellite (patents) Documents 1 and 2) are widely used.

  Nickel or cobalt self-fluxing alloy containing high Cr (SFNi 4, SFNi 5, SFCo 1, SFCo 2) has excellent wear resistance, but has a high coefficient of thermal expansion such as austenitic stainless steel In addition, since the coating film coated by thermal spraying or overlay welding has low toughness, it cannot follow the expansion and contraction of the member, and cracking and peeling may occur, and the function of the device may be greatly deteriorated.

  Co-Cr-WC alloys represented by stellite have very high toughness, and coatings formed by thermal spraying and overlay welding are less prone to peeling and cracking. Although thermal spraying and overlaying are possible, there is a problem in wear resistance in a high temperature environment because of low hardness and lack of age hardening.

  As described above, currently used surface hardening alloys do not have materials that combine heat resistance, wear resistance, high toughness and age hardening, and are particularly resistant to austenitic stainless steels used in high temperature environments. It is difficult to find a surface hardening alloy for the purpose of imparting wear.

JP 2001-123238 A Special table 2008-522039

  In recent years, in order to reduce the environmental load, improvements in combustion efficiency and fuel consumption, and longer life of members and equipment have been promoted in various industrial fields. In particular, in superheaters and reheaters for coal-fired thermal power generation boilers, coating with a surface hardening alloy is being studied for the purpose of extending the service life, but austenitic stainless steel pipes are used as boiler tube base materials. However, even if this substrate is coated, there is no problem of peeling or cracking of the coating layer, and development of a surface hardening alloy having sufficient wear resistance has been an issue.

In the present invention, in the study of the alloy composition for developing a material having heat resistance, wear resistance and high toughness, and having age-hardening properties, the following targets are set, and all of them must be satisfied. Condition.
[Target value]
(1) Heat resistance → Co-based alloy containing 25.0% by mass or more of Cr.
(2) Abrasion resistance → Rockwell hardness (HRC) must be 44 or more.
(3) High toughness → Charpy impact value must be 15 J / cm ² or more.
(4) Age-hardening → Vickers hardness (HV) increases by 100 or more in heat treatment at 650 ° C for 1000 hours compared to before heat treatment.

Co-base alloy for surface hardening of the present invention that satisfies all of the above goals (1) to (4), the composition of which Cr is 25.0 mass% or more and 45.0 mass% or less, Fe is 8.0 mass% or more and 35.0 mass% or less, Si is contained in an amount of 1.0% to 5.0% by mass, B is contained in an amount of 0.2% to 2.0% by mass, and the balance is 30.0% to 60.0% by mass of Co and inevitable impurities. Furthermore, the surface-hardening Co-based alloy of the present invention has excellent age-hardening properties by having the above composition.
Here, inevitable impurities are impurities that are inevitably mixed in the manufacturing process of each raw material, although not intentionally added, and such impurities include Mg, S, O, N , V, Zr, Sn, etc., and the total amount thereof is usually 0.3% by mass or less and does not affect the properties of the alloy of the present invention.

  Further, the present invention is characterized in that the surface hardening Co-based alloy having the above-described characteristics contains at least one of W and Mo in a total amount of 6.0% by mass or less.

  Further, the present invention provides a surface-hardening Co-based alloy having the above-described characteristics, as additive elements that do not affect the characteristics, Ni is 12.0 mass% or less, Cu, Mn is 3.0 mass% or less, Al, Ti, Nb Is 1.0 mass% or less, C is 0.2 mass% or less, and the total amount of Ni, Cu, Mn, Al, Ti, Nb, and C is 12.0 mass% or less.

  Next, the reason why each component of the surface hardening Co-based alloy according to the present invention (hereinafter referred to as “the present invention alloy”) is limited will be described.

  Co-based alloys generally have better impact properties than Ni-based alloys and were selected as the base metal for the alloys of the present invention. However, if the amount of Co is less than 30.0% by mass, a Co-rich solid solution cannot be obtained, and the impact characteristics are greatly deteriorated. Further, if it exceeds 60.0% by mass, the characteristics to be imparted by other additive elements, that is, heat resistance and wear resistance cannot be obtained, so Co was set in the range of 30.0% by mass to 60.0% by mass.

  In addition to solid solution in the Co solid solution, Cr partly combines with B to crystallize chromium boride, which is an intermetallic compound, and imparts heat resistance and wear resistance, but is less stable than 25.0% by mass. No heat transfer film is formed and heat resistance is reduced. On the other hand, if it exceeds 45.0% by mass, a Cr-rich solid solution is crystallized or a coarse chromium boride is formed, and the strength (impact value and hardness) is greatly reduced. For this reason, Cr was set in the range of 25.0 mass% or more and 45.0 mass% or less.

  Fe stabilizes the fcc structure by dissolving in a Co solid solution, and causes a phase transformation from the fcc structure to the hcp structure in a high temperature environment. By this transformation, the alloy of the present invention has the property of age hardening to improve the wear resistance. However, if it is less than 8.0% by mass, a stable fcc structure cannot be obtained, and therefore phase transformation (age hardening) does not occur. On the other hand, if it exceeds 35.0% by mass, Fe dissolved in the Co solid solution becomes rich and heat resistance is lowered. For this reason, Fe was determined in the range of 8.0 mass% or more and 35.0 mass% or less.

  Si is dissolved in Co solid solution to improve the heat resistance and wear resistance of the matrix, and by adding it together with B, it gives self-fluxing and improves workability in thermal spraying and overlay welding. . However, if it is less than 1.0% by mass, the heat resistance and wear resistance are insufficient, and satisfactory workability cannot be obtained. On the other hand, if it exceeds 5.0% by mass, cobalt silicide of the intermetallic compound crystallizes, and the strength (impact value) is greatly reduced. For this reason, Si was determined in the range of 1.0 mass% or more and 5.0 mass% or less.

  B forms an intermetallic compound with Cr and imparts wear resistance, and by adding it in combination with Si, imparts self-fluxing and improves workability such as thermal spraying and overlay welding. If it is less than mass%, the wear resistance is insufficient and satisfactory workability cannot be obtained. On the other hand, if it exceeds 2.0% by mass, the chromium boride coarsens and the strength (impact value) is greatly reduced. Therefore, B is set in the range of 0.2% by mass or more and 2.0% by mass or less.

  W and Mo are dissolved in a Co solid solution to improve wear resistance, improve high-temperature strength characteristics, and further promote age hardening. However, when the total amount of these components exceeds 6.0% by mass, coarsening of the chromium boride is induced, and a significant reduction in strength (impact value) is caused. Therefore, the content of at least one of W and Mo is set to a range of 6.0% by mass or less.

  In the alloy of the present invention, as additive elements that do not affect the characteristics, Ni is 12.0 mass% or less, Cu, Mn is 3.0 mass% or less, Al, Ti, Nb is 1.0 mass% or less, C is 0.2 mass% or less However, in order not to impair the thermal shock resistance in particular, the upper limit of the total amount of Ni, Cu, Mn, Al, Ti, Nb and C was set to 12.0% by mass.

Since the alloy of the present invention has the following characteristics, it can be applied to a wide range of applications in addition to a coating material for a boiler tube for coal-fired thermal power generation.
(1) Co-based alloy containing 25.0% by mass or more of Cr and excellent in heat resistance.
(2) It has excellent wear resistance due to Rockwell hardness (HRC) of 44 or more.
(3) The Charpy impact value is 15 J / cm ² or more, and it has high toughness as a surface hardening alloy.
(4) Due to age-hardening, the wear resistance is further improved in a high temperature environment.

  The alloy of the present invention is prepared by adjusting and blending the base Co and the additive components Cr, Fe, Si, and B, so that W, Mo, Ni, Cu, Mn, etc. become a predetermined mass% as necessary. After the added metal is completely melted in the crucible of the melting furnace, the molten alloy is powdered by the atomization method or melt pulverization method, or cast into a predetermined mold to obtain a rod shape or plate shape Can do.

  In particular, the alloy powder produced by the atomizing method can be applied to surface modification using a thermal spraying method or a powder overlaying method by adjusting the particle size to be suitable for the intended thermal spraying method.

  The Example alloy and Comparative Example alloy of the present invention prepared and mixed as described above were melted, and Charpy impact value, Rockwell hardness, and age-hardening property were evaluated by the following methods.

(1) Charpy impact test: 100g of ingot with the composition of each alloy was heated and melted at about 1600 ° C in an argon stream using an electric furnace, cast into a shell mold, and described in JIS Z 2242: 2005 The test piece (without notch) was processed. And the impact test based on JIS Z 2242: 2005 was done using the Charpy impact tester.

(2) Rockwell hardness test: Cast pieces melted by the same method as in (1) above are ground into a square shape to give a parallel surface, and the upper portion of the parallel surface is compliant with JIS Z 2245: 2005 The test was conducted. The measurement was performed on the C scale.

(3) Age-hardening evaluation test: First, a test piece prepared by the same method as in (1) above is finally polished with a 1500-thick water-resistant abrasive paper and specified in JIS Z 2244: 2009 using a micro Vickers hardness tester A hardness test was performed and a reference hardness was measured. Next, hold the test piece in a muffle furnace set at a predetermined temperature (650 ° C) for 1000 hours, and after removing the oxide film from the test piece that was allowed to cool to the air and brought to room temperature, use a 1500 water-resistant abrasive paper. After final polishing, the hardness was measured in the same manner, and the hardness after the heat treatment was measured. And the thing with which the raise degree of the hardness before and behind heat processing was recognized 100 or more was determined to be "age hardening".

  Table 1 shows examples of the present invention, and Table 2 shows comparative examples.

  Comparative Examples 1 to 5 are surface-hardening alloys conventionally used in high-temperature environments, and Comparative Examples 1, 2, and 3 are self-fluxing alloys (SFNi 4, 5, SFCo 2) described in JIS H 8303: 2010 Comparative Examples 4 and 5 are Co—Cr—W alloys represented by stellite. In either case, the impact value or hardness was not satisfied, and no age hardening was observed.

  Comparative Examples 6 to 23 are alloys outside the claimed range of the alloys of the present invention. Comparative Example 10, Fe is outside the upper limit of the claim, Co is further outside the lower limit of the claim, Comparative Examples 11 and 12 are those Si is outside the upper and lower limits of the claim, Comparative Example 13, 14 is that B is outside the upper and lower limits of the claims, Comparative Example 13 is further Co is outside the upper limit of the claims, Comparative Example 15 is that Mo exceeds the upper limit of the claims, Comparative Example 16 is W and Mo In Comparative Examples 17 to 23, Ni, Mn, Cu, Al, Ti, Nb, and C exceeded the upper limit of the claims. In Comparative Examples 6 and 7, the age-curing property did not satisfy the target, and in Comparative Examples 8 to 23, the impact value or hardness did not satisfy the target value.

  On the other hand, as is clear from Table 1, Examples 1 to 20 which are the alloys of the present invention satisfied all of the impact value, hardness and age-hardening properties. As a result, the alloy of the present invention has wear resistance and high toughness, and has age-hardening properties, so that the wear resistance is further improved in a high temperature environment.

  As described above, the age-hardenable Co-based alloy that is suitable for surface hardening with excellent wear resistance according to the present invention has particularly high toughness, and therefore, a member or austenite that requires impact resistance characteristics. It is effective as a surface hardening alloy for members with a large coefficient of thermal expansion, such as stainless steel, and is wear resistant and age hardened, so it can be applied to a wide range of applications.

  The alloy of the present invention can be applied not only to a superheater / reheater of a boiler tube for thermal power generation but also to surface hardening of an engine valve.

  The alloy of the present invention can be used for a wider range of applications by adapting to various surface modification processes such as overlay welding and thermal spraying and sintering of HIP and the like.

Claims (3)

  Cr 25.0% to 45.0% by mass, Fe 8.0% to 35.0% by mass, Si 1.0% to 5.0% by mass, B 0.2% to 2.0% by mass, and the balance 30.0% by mass A Co-base alloy for surface hardening, characterized by comprising 60.0% by mass or less of Co and unavoidable impurities.   The surface hardening Co-based alloy according to claim 1, comprising at least one of W and Mo in total of 6.0 mass% or less.   Ni is 12.0 mass% or less, Cu, Mn is 3.0 mass% or less, Al, Ti, Nb is 1.0 mass% or less, C is 0.2 mass% or less, Ni, Cu, Mn, Al, Ti, Nb, C 3. The surface hardening Co-based alloy according to claim 1, wherein the total amount is 12.0% by mass or less.