Effect of severe carburizing conditions on superalloys

The influence of a strong carburizing condition was analyzed for 5 different alloys in the coal gasification conditions. The analyses were performed at temperatures 1330 – 1808oF at 68 atm for 80 hours. The tested alloys were Incoloy 800, Stainless steel 310,  18-8-2 and alloy 671. The test gas conditions include H2S, CO, CO2, H2, CH4 and H2O.

It is found that temperature has a considerable influence on the behavior of alloy. Normally, every alloy produced a security layer at a temperature range of 1300oF to 1400oF and internal sulfidation particularly across the grain boundaries at temperatures up to 1600oF to 1800oF. No sign of carburization occurred on any alloy.

Scale production

It is a surface mechanism that occurs through a chemical reaction between the gas conditions and the metal structure. The scale can be sturdy or unstable, regular or intermittent. When the sturdy layer becomes a regular, non-porous film, the received security layer can be useful. Spalling scale is unwanted. Sulfide or oxide scales can be produced in the coal conversion conditions. The sulfide reaction happened more rapidly. The partial pressures of sulfur in a coal gasification condition may vary from 10(5)  to 10(7) atm whilst the oxygen partial pressures is often lower to 10 (15) to 10(20) atm. The sulfidation and oxidation reactions may take place internally and at the surface. The internal reactions produce products that are named as sub-scale. The internal chemical reactions are complicated in nature which may interact with oxygen, sulfur, aluminum, titanium, chromium, manganese, silicon, nickel, iron etc. Usually the internal sulfidation or oxidation reaction products emerge as precipitations around the grain lines.

The commercial alloys that are subjected to 100 hour increments to a general coal gasification conditions comprising of 1% hydrogen sulfide and 40% water vapor produce sturdy oxide layers and grain limits oxides. Stainless steel 310 produced a spalling oxide and is attacked by internal sulfidation. It may be possible that a higher water vapor concentration resulted to produce an adherent oxide that enhanced spalling resistance while the thermal cycling and avoided internal sulfidation.

In other test the alloys were subjected to gas conditions of 41% argon, 16% water and partial pressure of oxygen about 2.4 x 10(6), at the temperature of 1800oF for 100 hours. Now few alloys produced sturdy oxide films whilst others produced spalling oxides. The alloy samples were pre-oxidized at 1800 – 2300oF in air or argon (Ar) containing water vapor for 8 – 16 hours and then subjected to 0.1 atm sulfur vapor at 1000oC. An influence of pre-oxidation was significant for alloy 956 however not for Fe13Al.

Stainless steel 310 and Incoloy 800 were analyzed at temperature 1800oF for 19 to 24 hours in a media of variable water and hydrogen partial pressure ratio. Grade 310 shows an unlike reaction from production of a liquid sulfide on the surface to the production of surface oxide and internal oxidation without sulfidation. Incoloy 800 and stainless steel 310 grades in a combined gas condition at 1382oF to 1832oF are noticed. At constant sulfur pressure, scale thickness and penetration of Incoloy 800 caused to reduce with enhanced oxygen partial pressure. The mechanisms of weight gain for alloys subjected to a combined gas condition for about 160 hours at 1600oF is observed. Hastelloy C276 and Haynes 188 attained the minimum weight after exposure for 160 hours.

Alloy selection

Alloy designing programs show an essential source of data for the selection of construction materials for coal conversion units. An objective of this program for materials for internal components is to choose materials that have sufficient tensile characteristics with high corrosion resistance. Resistance to attack received from the production of regular adherent surface layers, often oxides that prevent the internal diffusion of sulfur and subsequent sulfur compound production. The development of rigid security layer on several structural alloys is empowered by the availability of minimum 20% water vapor in gas conditions at temperatures about 1800oF.

A complete alloy program was conducted for which above hundreds of alloys were tested to find the influence of chemistry and nominal alloying inclusion on the resistance to sulfidation. Hot gas corrosion analyses were performed in an artificial coal gasification conditions comprising of 1 – 1.5% hydrogen sulfide and around 40% water vapor. The test periods varies from 25 to 500 hours. The temperature and pressure limits taken were 1800oF and 1 at. The alloys chosen were based on nickel, chromium and aluminum systems. The corrosion service of several iron based alloys was analyzed including the weight change and corrosion extracts produced. In various cases, the samples were fully damaged.

Heanjia Super-Metals test the samples before deliver for service to ensure their functionality in the required application media. Our products are ISO certified. Contact us to get more information on the various corrosion resistant super alloys to choose a specific one for your application.