Cause of high creep and rupture strength of incoloy 800H and incoloy 800HT

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Cause of high creep and rupture strength of incoloy 800H and incoloy 800HT

The carbon content present in the incoloy 800 wire is proportion to creep and cracking properties of alloy such that more carbon content, more are the creep and cracking properties. Therefore carbon is added significantly in the alloy on the special application requirements. The carbon is added by 0.05% to 0.10% in the ASTM and ASME limits.

The manufacturers of incoloy 800 came in the alloy market had used different content of elements providing different values of creep and rupture. Some have introduced alloy with comparatively lower strength. The standard property values are determined at the temperatures of 1500oF (593-816oC), and 1650oF (871 and 899oC). The concentration of aluminum and titanium ensure the higher creep and stress rupture characteristics as compare to incoloy 800H and the new alloy with enhanced concentration of aluminum and titanium is called as incoloy 800HT. It is heat processed at 2100oF or 1149oC. The concentration of carbon is 0.06 to 0.10% in incoloy 800HT whereas it is 0.05 to 0.10% in incoloy 800H. The content of aluminum and titanium is 0.85 to 1.20% however in incoloy 800H, it is 0.30 to 1.20%.

The highest permitted stress of incoloy 800HT is included in ASME code case 1987. it meets the entire criteria for UNS N08811 and N08810 made for incoloy 800H and it can be authenticated to both or single UNS codes. It is essential to note that incoloy 800HT wire offers more permit-able design stress as compare to UNS N08810. That's why the other materials manufactured to UNS N08810 refers to incoloy 800H can not be authenticated for UNS N08811 code without meeting the criteria for this grade. The incoloy 800HT is product of extended analysis and maintenance of the ultimate features of incoloy series alloys.

The chemical composition of incoloy series alloys is provided below:

Element	Incoloy 800	Incoloy 800H	Incoloy 800HT
Ni	30.0-35.0	30.0-35.0	30.0-35.0
Cr	19.0-23.0	19.0-23.0	19.0-23.0
Fe	39.5 min.	39.5 min	39.5 min
C	0.10 max	0.05-0.10	0.06-0.10
Al	0.15-0.60	0.15-0.60	0.25-0.60
Ti	0.15-0.60	0.15-0.60	0.25-0.60
Al+Ti	0.30-1.20	0.30-1.20	0.85-1.20
ASTM grain size	Not mentioned	5 or coarser	5 or coarser

The special element requirements of incoloy 800H are mentioned in the below table:

C	0.08 max.
Al + Ti	0.4-0.7
ASTM grain size	Unique

The special grain size criteria of incoloy alloys provided following:

	INCOLOY alloy 800H & 800HT
Plate	ASTM 1-5
Tube/Pipe	ASTM 1-5
Sheet	ASTM 2-5

The incoloy 800H and 800HT offer extremely higher creep and cracking strength properties as compare to incoloy 800. These alloys possess almost similar chemical composition such that the content % of the basic elements like nickel, chromium and iron is same. But, the composition varies in terms of concentration of carbon, titanium and aluminum. The concentration of carbon in incoloy 800 is 0.10%. The chemical composition of incoloy 800HT is more limited as compare to 800H. The concentration of carbon in incoloy 800HT is 0.06 – 0.10% and aluminum + titanium concentration is 0.85 – 1.20%. The concentration of elements in the incoloy 800HT lies always within the limits of alloy 800H. However the concentration limits of incoloy 800H wire may or may not reside within the limits of 800HT alloy.

Besides of the limited concentration of carbon, both 800H and 800Ht alloys are annealed at the elevated temperatures to obtain the suitable grain size of level ASTM five. The annealing and limited chemical content% offer the incoloy alloys superior creep and rupture forces.

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