Type Analysis | Description | Corrosion Resistance | Physical Properties | Heat Treatment | Workability | Typical Mechanical Properties

Type Analysis

Description

Alloy 350 is a chromium-nickel-molybdenum stainless steel which can be hardnened by martensitic transformation and/or precipitation hardening. It has been used for gas turbine compressor components such as blades,discs,rotors,and shafts,and similar parts where high strength was required at room and intermediate temperatures. Depending upon the heat treatment,alloy 350 may have an austenitic structure for best formability,or a martensitic structure with strengths comparable to those of martensitic steels. The alloy normally contains about 5 to 10% delta ferrite. The corrosion resistance of alloy 350 approaches that of the chromium-nickel austenitic stainless steel

Corrosion Resistance

Alloy 350 has corrosion resistance superior to that of other quench-hardenable martensitic stainless steels. It has shown good corrosion resistance in ordinary atmospheres and numerous other mild chemical environments. Material in the double-aged or equalized condition is susceptible to intergranular corrosion because of the precipitation of chromium carbides. When the alloy is hardened by treatments employing sub-zero cooling as in the following paragraph,it is not subject to intergranular attack. The treatment for optinum stress-corrosion resistance of alloy 350 is as follows:
Heat to 1850/1950 °F(1010/1066 °C),cool rapidly to room temperature,sub-zero cool 3 hours at -100 °F(-73 ° C) reheat to 1700/1750 °F(927/954 °C)about 90 minutes per inch(25.4 mm)of thickness,cool rapidly to room tempertature,sub-zero cool 3 hours at -100 °F(-73 °C),then temper 3 hours at 1000 °F(538 °C). For optimum corrosion resistance,surfaces must be free of scale and foreign particles and finished parts should be passivated.

Physical Properties

Electrical resistivity
Sub-zero cooled,tempered 850 °F(454 °C)

Mean Coefficient of Thermal Expansion
Sub-zero cooled,tempered 850 °F(454 °C)

Thermal Conductivity
Sub-zero cooled,tempered 850 °F(454 °C)

Moduli of elascity (E) and Rigidity (G)

Heat Treatment

Annealing
Heat to 1850/1950 °F(1010/1066 °C),cool rapidly to room temperature.

Hardening
Alloy 350 can be hardened by either sub-zero cooling and tempering (SCT) or double aging(DA). Sub-zero cooling and tempering will result in higher strength than double aging. "Conditioning" of the alloy by rapid cooling from 1710 °F (932 °C) +/-25 °F is required before the SCT treatment,and is not required, but is recommended before double aging. It is further recommended that following an anneal at 1850/1950 ° F (1010/1066 °C), alloy 350 be cooled to -100 °F (-73 °C) for at least 3 hours before hardening.

Sub-zero cooling
After conditioning at 1710 °F(932 °C) +/-25 °F (rapid cool) for 90 minutes per inch of thickness, alloy 350 is held for a minimum of 3 hours at -100 °F, then tempered at either 850 °F or 1000 °F (454 °C or 538 °C) for a minimum of 3 hours. The 850 °F temper produces the highest strengths and hardnesses,and the 1000 °F temper produces improved toughness and stress corrosion properties.

Double age
Hold for 3 hours at 1350/1400 °F(732/760 °C), air cool to room temperature; heat to 825/875 °F(440/468 ° C), hold for 2-3 hours, air cool.

Workability

Hot working
Alloy 350 is readily hot worked. It is worked from a maximum temperature of 2150 °F(1177 °C). The use of temperature above 2150 °F will cause an increase in the amount of ferrite. Finishing temperature should be in the range of 1700/1800 °F(927/982 °C) to prevent grain coarsening on subsequent heat treatment and promote homogenous precipitation of carbides.

Cold working
In the annealed condition, alloy 350 is essentially austenitic and has forming characteristics similar to those of the AISI 300 series stainless steels. It has a higher rate of work hardening and cold forming will cause martensite formation in proportion to the amount of deformation. If capacity is limited or deformation is severe, heating the material to 300 °F(149 °C) or above will minimize work hardening. In the hardened condition, alloy 350 has sufficient ductility for limited forming or straightening operations .

Machining
Succesfully maching alloy 350 requires the same pratices used for other stainless steels, such as rigid tool and work suports, slower speeds, positive cuts, absence of dwelling or glazing, and adequate coolant. In the annealed condition, the alloy is soft and gummy and has a high work-hardening rate. Machining alloy 350 in the annealed condition is consequently not recommended. Best machinability is obtained in the equalized and overtempered condition. Finishing operations may be performed in this condition if proper allowances are made for growth during subsequent hardening treatments. If extreme dimensional accuracy is necessary, finish machining should be done in the hardened condition.
Following are typical feeds and speeds for equalized and overtempered alloy 350:

When using carbide tools, surface speed feet/minute (sfpm) can be increased
between 2 and 3 times over the high-speed suggestions. Feed can be increased
between 50 and 100%.
Figures used for all metal removal operations covered are average. On certain
work, the nature of the part may require adjustment of speeds and feeds. Each
job has to be developed for best production results with optimum tool life.
Speeds or feeds should be increased or decreased in small steps.

Welding
Alloy 350 can be satisfactorily welded by the shielded fusion and resistance welding processes. Oxyacetylene welding is not recommended, since carbon pickup in the weld may occur. When a filler metal is required, a machining analysis should be used to provide welds with properties approximately the same as the base metal. When designing the weld joint , care should be exercised to avoid stress concentrators, such as shrap corners, threads,and partial-penetration welds. When high weld strength is not needed, a standard austenitic stainless filler, such as E/ER308, should be considered. Preheating is not required to prevent cracking. If possible, the weldment should be annealed after welding to provide the optimum combination of strength, ductility, and corrosion resistance. The alloy must be treated at 1710 °F(932 °C) before hardening by sub-zero cooling and tempering.

Typical Mechanical Properties

Typical Room Temperature Mechanical Properties

Typical Elevated Temperature Tensile Properties
Sub-zero cooled, tempered 850 °F(454 °C)

Typical Stress Rupture Strength
Sub-zero cooled, tempered

Typical Room Temperature Tensile Properties After Exposure to
Elevated Temperature Under Stress
Sub-zero cooled, tempered 850 °F(454 °C)

*Broke outside gage marks