Ni-Cr-Fe

(Andrew Johnson>

The ternary diagram of Ni-Cr-Fe is one of the most used ternary diagrams in history. The two types of alloys covered here from the Ni-Cr-Fe diagram are Stainless Steel and Inconel.

Stainless Steel

There are a vast number of types of Stainless steel. A metal alloy with a Chromium content greater than 11.5 % and an Iron content greater than 50 % is called a stainless steel. The stainless steels are broken into three major classes:

1. Chromium (11.5-17%) -iron alloys with carefully controlled carbon content. Can be heat treated to a magnetic martensite structure and are therefore known as martensitic stainless steels.

2. Chromium (17-27%) -iron alloys with low carbon content. Nonhardenable by heat treatment. Their crystal structure is magnetic ferrite and therefore are known as ferritic stainliess steels.

3. Chromium (16-26%) Nickel (6-22%) -iron alloys with low carbon content. Nonhardenable by heat treatment. Crystal structure of nonmagnetic austenite so are therefore called austenitic stainless steels.

The following table compares the composition and some basic properties of various types of stainless steel.

A Comparison of Stainless Steels
Type of
Steel 		%
Nickel 		%
Chromiium 		%
Carbon 		%
Manganese 		%
Silicon 		%
Nitrogen 		Corrosion
mpy (1) 		Melting
Range °F 		Young's Modulus
x 10^6,psi
201 3.5 - 5.5 16.0 - 18.0 0.15 max 5.5 - 7.5 1.0 max 0.25 max 20 2550 - 2650 28.0
301 6.0 - 8.0 16.0 - 18.0 0.15 max 2.0 max 1.0 max 0 12 2550 - 2590 28.0
302 8.0 - 10.0 17.0 - 19.0 0.15 max 2.0 max 1.0 max 0 10 - 18 2550 - 2590 28.0
304 8.0 - 10.5 18.0 - 20.0 0.08 max 2.0 max 1.0 max 0 6 - 12 2550 - 2650 28.0
309 19.0 - 22.0 24.0 - 26.0 0.2 max 2.0 max 1.0 max 0 5 - 9 2550 - 2650 29.0

(1) Corrosion rate is in mils per year. The test was performed in 65% Nitric acid at 245 °F.

(2) Table references

Types of Stainless steel and specific uses of each.

Inconel

Inconel is a specialty alloy that uses higher percentages of Nickel and Chrome than Stainless steel, as well as many other elements in small quantities. These small additions of other elements is solid-solution hardening. It is quite expensive and therefore usually reserved for applications when some type of stainless steel won't suffice. The following tables compare Inconels properties.

A Structural Comparison of Inconel Alloys
Type %
Ni 		%
Cr 		%
C 		%
Mn 		%
Si 		%
Fe 		%
S 		%
Cu 		%
Al 		%
Ti 		%
P 		%
Co 		%
Nb 		%
B 		%
Mo
600 72.0 min 14.0
-
17.0 		0.15 max 1.0 max 0.5 max 6.0
-
10.0 		.015 max 0.5 max 0 0 0 0 0 0 0
601 58.0
-
63.0 		21.0
-
25.0 		0.1 max 1.0 max 0.5 max bal .015 max 1.0 max 1.0
-
1.7 		0 0 0 0 0 0
625 58.0 min 20.0
-
23.0 		0.1 max 0.5 max 0.5 max 5.0 max 0.015 max 0 0.4 max 0.4 max .015 max 1.0 max 3.15
-
4.15 		0 8.0
-
10.0
718 50.0
-
55.0 		17.0
-
21.0 		0.08 max 0.35 max 0.35 max bal .015 max 0.3 max 0.2
-
0.8 		0.65
-
1.15 		.015 1.0 max 4.75
-
5.5 		.006 max 2.8
-
3.3
800 32.5 21.0 0.1 max 0.8 max .008 max 46.0 0 0.4 0.4 0.4 0 0 0 0 0

(1) Table references

Corrosion of Inconel

One outstanding characteristic of high-nickel alloys, like Inconel, is their good resistance to a wide variety of corrosives. With few exceptions, high-nickel alloys do significantly better than martensitic, ferritic, and austenitic stainless steels in corrosive environments.

A Comparison of Inconel Properties
Type Yield Strength
70°F
ksi 		Melting
Range °F 		Rupture Strength; 100h at ____°F, ksi
600 41.3 2471 - 2579 1600, 5.3
601 49.0 2471 - 2579 1600, 7.0
625 71 2300 - 2435 1600, 10.5
718 172 2300 - 2435 1200, 100
800 36.3 2471 - 2525 1800, 21

(1) Table references

The uses of Inconel are specific and quite costly:
This page by Andrew Johnson