Overview of Hastelloy C series alloys
Ni-Cr-Mo alloy is a Hastelloy C series alloy, mainly a nickel-based alloy; This type of alloy also includes Inconel 625, Inconel 686, VDM59, etc. The development, material properties, corrosion properties, welding material hot processing, etc. of Hastelloy C series alloys are briefly introduced.
1. Alloy Overview
Hastelloy alloys are divided into corrosion-resistant alloys and heat-resistant alloys; Corrosion-resistant alloys are divided into three major series, namely B, C, and G. Hastelloy B series alloys, B-2, B-3; Hastelloy C series alloys: C, C-276, C-4, C-22, C-2000; Hastelloy G series alloys: G, G-3, G-30, G-50, etc. Hastelloy alloys are corrosion-resistant alloys, and the most widely used is C alloy.
In the 1920s, the first C alloy was Hastelloy C. In the second half of the 20th century, corrosion-resistant alloys have made great progress. In the 1960s, there was C-276, in the 1970s, there was C-4, in the 1980s, there was C-22, and in the 1990s, there were alloys 59, 686, C-2000, etc.
Today, the companies producing Hastelloy C series alloys internationally are mainly Haynes International, Inc. in the United States (Haynes International, the research and development company of Hastelloy C series alloys) and Special Metals Corporation (SMC High Temperature Alloy Group) in Germany and ThyssenKrupp VDM (ThyssenKrupp VDM) in Germany. The comparison of commercial grades of C series alloys produced by different companies is shown in Table 1.
1.1 Hastelloy C
Hastelloy C is formed by adding Cr and W elements to Hastelloy B alloy. It is the compatibility and optimization of Ni-Cr alloy and Ni-Mo alloy. It has good corrosion resistance and corrosion resistance in oxidizing and reducing media. Local corrosion, resistance to chloride stress corrosion cracking and seawater pore corrosion.
Hastelloy C and Hastelloy B also have some serious disadvantages; in harsh oxidizing media, the chromium content of this alloy is not enough to keep it passivated and exhibits a high uniform corrosion rate, which poses a greater obstacle to the application of the weld heat-affected zone in many oxidizing, low pH, halide environments. It is very sensitive to intergranular corrosion. Many occasions require that containers made of Hastelloy C alloy must be solution treated after welding to eliminate segregation in the heat-affected zone, which seriously limits the application of this alloy. In addition, the solution treatment process will significantly reduce the plasticity and impact toughness of Hastelloy C alloy. Hastelloy C has been eliminated except for the use of certain casting materials.
1.2 Hastelloy C-276
In Hastelloy C-276 The biggest obstacle to the development of C alloy before it appeared was the need for post-weld solution treatment, and welding is a necessary process for the manufacture of most equipment. Welding greatly reduces the corrosion resistance of the weld and heat-affected zone, and Hastelloy C-276 provides a solution to this problem. Due to the extremely low C and Si content, the corrosion resistance of the weld heat-affected zone is almost the same as that of the parent material. C-276 was introduced in 1965 and quickly became one of Haynes' leading products. In many corrosive environments, the corrosion resistance of alloys C and C-276 is similar. C-276 alloy does not have continuous grain boundary segregation in the heat affected zone of the weld, so it will not cause severe intergranular corrosion.
C-276 can be used in the welded state, but even low-carbon and low-silicon C-276 is sensitive to intergranular corrosion under certain process conditions; C-276 does not have sufficient thermal stability after long-term aging in the temperature range of 650-1090℃, and it will also precipitate carbides at the grain boundaries or produce intermetallic compound μ phase (Co2Mo6 type), which reduces the intergranular corrosion resistance. To overcome this sensitivity, Hastelloy C-4 was developed in the 1970s with better high-temperature stability.
1.3 Hastelloy C-4
Hastelloy C-4 has remarkable high temperature stability when placed at 650-1,040℃ after long-term aging, showing good ductility and resistance to intergranular corrosion. The formation of grain boundary deposits can be resisted in the heat-affected zone of the weld.
In alloy C-4, in addition to the significant reduction in C and Si content, the main changes are the removal of tungsten from the basic chemical composition and the reduction of iron-titanium additions. This composition adjustment significantly improves thermal stability and eliminates the precipitation of intermetallic compounds and grain boundary segregation in the alloy. Alloys C-276 and C-4 have basically the same overall corrosion resistance in many corrosive environments, with alloy C-276 performing slightly better in strong reducing media such as hydrochloric acid, while alloy C-4 has excellent corrosion resistance in highly oxidizing media.
In highly oxidizing environments, C-4 and C-276, which contain only 16% chromium, cannot provide effective corrosion resistance, and the development of other alloys such as C-22 and VDM59 has overcome this shortcoming.
1.4 HASTELLOY C-22
Alloy C-22 was introduced in 1982 when the patent registered in the United States for alloy C-276 expired. Alloys C-276 and C-4 corroded rapidly in oxidizing non-halogenated solutions because they had the lowest chromium content of the C alloys. Oxidizing environments require a high chromium alloy with an optimized balance of Cr, Mo, and W, resulting in an alloy with high corrosion resistance and good thermal stability. This guiding principle led to the creation of HASTELLOY C-22, an alloy whose chromium, molybdenum, and tungsten contents were carefully adjusted to achieve current levels of resistance to oxidizing acid corrosion while meeting the need for high temperature stability. Although the alloy's corrosion resistance in highly oxidizing environments is superior to that of alloys C-276 and C-4, it does not perform as well as alloys C-276 and 59 in strongly reducing environments and severe crevice corrosion conditions, as alloys C-276 and 59 contain 16% molybdenum. Alloy C-22 is often used in the corrosive environments of flue gas desulfurization systems and complex pharmaceutical reactors.
