Nov 23 2012
Topics Covered
IntroductionChemical CompositionPhysical PropertiesMechanical PropertiesOther DesignationsFabrication and Heat Treatment Machinability Forming Welding Heat Treatment Forging Cold WorkingApplications
Introduction
Super alloys are also called high performance alloys. They have the capacity to function under extreme temperatures and severe mechanical stress, and also where high surface stability is required. They have good creep and oxidation resistance. Super alloys contain many elements in a variety of combinations so as to achieve the desired result. Super alloys can be strengthened by solid-solution hardening, work hardening, and precipitation hardening methods.
There are three types of super alloys, namely cobalt-base, nickel-base, and iron-base. All of these can be used at temperatures above 540°C (1000°F).
Super alloy 155 is an iron-based alloy with high strength resistance of up to 815°C (1500°F). It can sustain moderate loading at temperatures of 1093°C (2000°F). It has good oxidation resistance at high temperature and good corrosion resistance to sulfuric acid (at room temperature) and to nitric acid. This alloy’s high temperature strength is gained from its chemical composition rather than from age hardening.
The following datasheet will provide more details about super alloy 155.
Chemical Composition
The following table shows the chemical composition of super alloy 155.
| Element |
Content (%) |
| Chromium, Cr |
20-22.5 |
| Nickel, Ni |
19-21 |
| Cobalt, Co |
18.5-21 |
| Molybdenum, Mo |
2.5-3.5 |
| Tungsten, W |
2-3 |
| Manganese, Mn |
1-2 |
| Silicon, Si |
1 max |
| Niobium,Nb + Tantalum, Ta |
0.75-1.25 |
| Carbon, C |
0.08-0.16 |
| Phosphorus, P |
0.04 max |
| Sulfur, S |
0.03 max |
| Nitrogen, N |
0.1-0.2 |
| Iron, Fe |
Remainder |
Physical Properties
The physical properties of super alloy 155 are outlined in the following table.
| Properties |
Metric |
Imperial |
| Density |
8.19 g/cm³ |
0.296 lb/in³ |
| Melting point |
1343°C |
2450 °F |
Mechanical Properties
The mechanical properties of super alloy 155 are provided below.
| Properties |
Metric |
Imperial |
| Poisson’s ratio |
0.298 |
0.298 |
Other Designations
Equivalent materials to super alloy 155 are provided below:
- AISI 661
- AMS 5532
- AMS 5585
- AMS 5768
- AMS 5769
- AMS 5794
- ASTM B639
- DIN 2.4964
- GE B50A484
Fabrication and Heat Treatment
Machinability
Super alloy 155 behaves like the austenitic 300 series stainless steels during machining using conventional methods. Carbide and high-speed tool steels can be used for turning or boring. It is better to use high-speed steels work while performing drilling and milling operations.
Forming
As Super alloy 155 has good ductility, it can be easily formed using conventional methods. It has the tendency to work harden, hence may need a solution heat treatment to gain back the ductility during cold forming.
Welding
Super alloy 155 is easily weldable by resistance, metal inert gas welding (MIG), and tungsten inert gas (TIG) methods. It is not suitable to use submerged-arc welding for high temperature alloys as the high heat input during that process can cause cracking in the weld area. Pre-or post-heating is not required for super alloy 155.
Heat Treatment
Super alloy 155 can be solution heat treated at 1177°C (2150°F) and then water quenched or quickly air cooled.
Forging
Hot forging of super alloy 155 should be performed by heating the billet to 1177°C (2150°F) to allow through heating. It should be ensured that forging is stopped before the temperature drops below 927°C (1700°F). Re-heating can be performed in case additional forging is required.
Cold Working
Super alloy 155 can be cold worked and drawn easily. As work hardening occurs, it is suitable to perform solution heat treatment before completing all cold working.
Applications
Super alloy 155 can be applied in the following:
- Exhaust manifolds
- Industrial furnace structural parts and fittings
- High temperature bolts and gas turbine combustion chambers
- Turbine blades.