Debasis Heat resisting alloys - i.e., behaviour of these at temp 800-1000 deg cent - High temp metallurgy and alloy ratio in H series of alloys is a complicated phenomenon. It is also combined with the type of micro-structure required to get 100,000 hr creep-rupture strength for heat resisting materials. Ratios of Si to Mn, Ratios of Nb or rare earth elements from improving oxidation, carburization, mechanical properties play critical parts. So also the inter-metallic precipitates after long service exposures. Keeping an optimum ratio of alloying elements to maintain all these from desired micro-structure and mechanical properties - usually experienced by foundries of heat resisting alloy manufacture is compounded by availability of raw virgin material supplies in different countries. Q1: Mechanical properties in heat resistant grade alloys do deteriorate after continuous exposure after 20,000 hrs. But not very drastically if the furnace had been operated within design parameters - process, heat influx (burner configuration), and fuel used in the furnace, furnace draft ( ID & FD fan performances), cyclic shut downs the furnace had undergone during the first 3 years. Rapid deterioration - on set to tertiary stage - is what the user should be concerned rather than theoretical on-set of creep at 20,000 hrs. Usually, rapid on set to tertiary stages happen at much later stages of 45,000 to 90,000 hrs prolonged service for these materials. Mechanical properties drop to 1/3 or even lower than original values but not at first 3 years service life. Q:2- Micro structure inspection at manufacturing stage by manufacturer, compliance to P.O by III Party inspection or company;s own representative witnessing and certifying - setting parameters depends on the technical and purchase specifications of individual operating company including short term and medium term stress-rupture properties expected and witness the same during manufacturing stage. Nb/ C if specified in P.O it can be obtained in simple chemical analysis when the melt is taken out. Nb/C ratio may be difficult to evaluate unless special etchant conditions are stipulated at the manufacturing stage . This has to be carefully looked by the quality check teams (manufacturing, III Party inspection or company representative - who have knowledge to look inti such specific micro-structure aspects in P.O compliance) If such P.O. compliance is met, user gets benefit - no doubt- to such quality control at the manufacturing stages from long term use even beyond 100,000 hr service life in heat resistant alloys. Hope this helps you C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 17, 2007 E-mail: nishi@vsnl.com >High Temperature Super alloys contains "Si" to a reasonable >amount in order to protect MOC from surface breaking >Carburisation by generating impervious sublayer. But it >forms internal "Metal Silicides" ( giga and eta-prime >phases) w.r.t. exposures. These silicides in conjunction >with NbC causes significant embrittlement resulting in >reduction in impact strength. > >It has also been advised by metallurgists to keep "Si" as >low as possible by judicious increment of "Mn" to restore >melt fluididty at production stages and also to maintain a >stoichiometric composition @ Nb / C of 7.7. > >Q1. It has been experienced by many users that decrease in >Impact Performances & mech. props occured during 1st 3 years >of service with very little further change later on over >longer exposure ? Need a discussion / share please. > >Q2. Any comment of Nb/C ratio maintenance and users >prerogative to evaluate as a part of Receiving >Micro-Inspection looking into long-term perspective. >regards > >