NICKEL ALLOYS FABRICATION INSTRUCTIONS

FABRICATION INSTRUCTIONS FOR HIGH-ALLOYED AUSTENITIC STEELS AND NICKEL ALLOYS

High-alloyed austenitic steels and nickel-base alloys can be worked by all the conventional forming and machining processes. However, they are often more cost-intensive than in the case of the conventional structural steels, and this should be appropriately taken into account in the cost calculation.

1. FORMING

The term “cold working” is refer to the forming process which is carried out at room temperature or at well below the recrystallization temperature.

1.1 Cold working

Austenitic stainless steels and nickel-base alloys may be readily cold-worked, provided that the specific material properties are taken into consideration.

• initial value of 0.2 % proof stress
• work hardening
• decrease in elongation

Maximum permitted deformation in cold working without heat treatment

Cold working alters the mechanical properties of the materials.

• All the materials in question are prone to appreciable work hardening accompanied by a decrease in the elongation after fracture in tensile testing.
• Some materials, especially those containing Mo, are prone to precipitation if, following extensive cold working, welding is carried out in the zone of deformation. As a result, these zones may become sensitised, i.e. their corrosion resistance may be impaired.

Normally for nickel alloys, cold working up to 15 % deformation is permitted without subsequent heat treatment. In individual cases, though, depending on the material and the duty heat treatment may be necessary even if the degree of deformation is £ 15 %, e.g. when using solution heat treated high-temperature alloys. In other cases, a higher degree of deformation may be permitted without heat treatment, especially if no welding is to be carried out in the zone of deformation.

Tools and machinery

Forming of austenitic chrome-nickel steels and nickel-base alloys is very often carried out with the same tools, fixtures and machines that are used for forming of mild steels. Extraneous ferrite particles on the surface of these tools lead in service to corrosive attack which may destroy the component’s passive layer and thus locally decrease the corrosion resistance. Care should therefore be taken that tools, supports and the like are cleaned to such an extent that no abraded particles of mild steel are carried onto the high-alloyed work pieces.

The product-side surfaces of the finished components should be checked for absence of ferrite, or pickled and passivated, before delivery / entry into service.

1.2 Hot working

Hot working is carried out in a range between the recrystallization temperature and the solidus temperature. As a result, the resistance to deformation is greatly reduced and the forces required to work the material are correspondingly lower.

The following table shows, by way of example, the hot working and heat treatment temperatures for a number of materials and nickel-base alloys.

Alloy	Hot working temperature oC	Solution heat treatment temperature oC	Soft annealing temperature oC
926	900 – 1200	1150 – 1180	—
400	800 – 1200	—	700 – 900
800H	900 – 1200	1150 – 1200	—
825	900 – 1150	—	920 – 980
600	900 – 1200	(1080 – 1150)	920 – 1000
601	900 – 1200	(1100 – 1180)	920 – 980
617	950 – 1200	1150 – 1200	—
625	900 – 1200	—	950 – 1050
330	950 – 1180	1020 – 1120	—
C-276	950 – 1200	1100 – 1160	—
59	950 – 1180	1100 – 1180	—

During hot working, care should be taken that deformation proceeds as uniformly as possible so as to prevent the formation of an inhomogeneous grain structure. In addition, for low degrees of deformation (£ 30 % approx.), the forming temperature should be as close as possible to the lower limit in order to prevent coarse-grain growth. For higher degrees of deformation (> 30 % approx.), the higher temperatures are recommended.

After every hot working operation, heat treatment should be carried out in accordance with the mill product manufacturer’s instructions. Details with regard to heat control are described in the section Heat treatment.

2. HEAT TREATMENT

After hot working, the component should undergo heat treatment. After cold working, heat treatment may be unnecessary. The question of whether a finished component should be heat-treated should be settled with the client in each specific case, unless laid down in codes and specifications.

Heating

Work pieces must be clean and free from all kinds of contaminants before and during any heat treatment. Nickel alloys may become impaired if heated in the presence of contaminants such as sulphur, phosphorus, lead and other low-melting-point metals. Sources of such contaminants include marking and temperature-indicating paints and crayons, lubricating grease and fluids and fuels. Fuels must be as low in sulphur as possible. Natural gas should contain less than 0.1 wt.-% sulphur. Fuel oils with a sulphur content not exceeding 0.5 wt.-% are suitable. Due to their close control of temperature and freedom from contamination, thermal treatments in electric furnaces under vacuum or an inert gas atmosphere are to be preferred.

Treatments in an air atmosphere and alternatively in gas-fired furnaces are acceptable though, if contaminants are at low levels so that a neutral or slightly oxidizing furnace atmosphere is attained. A furnace atmosphere fluctuating between oxidizing and reducing must be avoided as well as direct flame impingement on the metal.

Work pieces made from materials with a high alloying content of molybdenum should be heated up rapidly. For heating, they should therefore be placed in a furnace which has already been heated to the desired temperature. Such materials include the 6% Mo steels and alloys 625, C-4, C-276 and 59

When the desired temperature has been reached, the following holding time is recommended as a guide:

For work piece thicknesses up to s = 10 mm              3 min/mm

For work piece thicknesses up to  s = 10 – 20 mm:       30 min plus 2 min/mm for

thicknesses > 10 mm

For work piece thicknesses    s = >20 mm:           50 min plus 1 min/min for

thicknesses > 20 mm

Cooling of high-molybdenum austenitic stainless steels and nickel-base alloys should be carried out rapidly so as to prevent undesirable precipitation. Delayed cooling, e.g. in the furnace, should be avoided at all costs, as this lead to the formation of precipitates, chiefly in the regions close to the grain boundary. Such precipitates may adversely affect both the corrosion resistance and the toughness properties of the material.

Proven results are obtained with a cooling rate of ³ 150 °C/min from the material-specific solution heat treatment temperature down to approx. 500 °C.

Heat treatment temperatures for a number of materials and nickel-base alloys

Alloy	Solution heat treatment temperature oC	Soft annealing temperature oC
926	1150 – 1180	—
400	—	700 – 900
800H	1150 – 1200	—
825	—	920 – 980
600	(1080 – 1150)	920 – 1000
601	(1100 – 1180)	920 – 980
617	1150 – 1200	—
625	—	950 – 1050
330	1020 – 1120	—
C-276	1100 – 1160	—
59	1100 – 1180	—

 

3. ABRASIVE BLASTING / PICKLING

In the case of components for wet-chemical service made from stainless steels and nickel-base alloys, it is usually necessary to remove the oxides formed during heat treatment.

For components intended for high-temperature service, the need for abrasive blasting/pickling should be agreed with the client

As the oxides adhere very strongly in the case of the higher-nickel materials, it is advisable to blast-clean the components with a suitable grit or with glass beads or to grind them with, for instance, 80 grit mop wheels prior to pickling.

Pickling is best carried out with commercial pickling pastes or by immersion in a pickling bath consisting of approx. 15-22 % nitric acid and approx. 2-3 % hydrofluoric acid.

Immersion times at room temperature (examples):

– Cr-Ni steels                                 2 –   8 hours

– Ni-Cr-Mo-Fe alloys                  8 – 24 hours

– Ni and Ni-Cu alloys                  10 – 15 minutes

– Ni-Mo alloys                              8 – 10 minutes

If the technical requirements, e.g. fume extraction, are met, it is recommended to increase the temperature of the bath to approx. 40 °C. This shortens the pickling time considerably.

The immersion time depends on the material, the oxide thickness and the temperature and should be tested at the start of the work.

In particular, the high-temperature materials with a fairly high C content as well as NiMo and NiCu alloys with a low Cr content are sensitive to over-pickling.

Any oxide still adhering after pickling should be removed with a chrome-nickel wire brush. The pickling operation should be repeated if necessary.

4. MACHINING

A high work-hardening rate and toughness and poor thermal conductivity are the main criteria applying to these materials. In machining, they should be taken into account by means of the following measures:

• Only use well-ground, sharp tools with smooth surfaces;
• Ensure maximum stability of tool, work clamping and machine tool in order to produce a clean cut;
• Apply plenty of coolant and lubricant;
• The tool should be constantly engaged, and with the relatively low cutting speed, a higher rather than too low a rate of feed should be used in order to prevent work-hardening of the material;
• If vibrations occur, the cause should immediately be established and remedied, because vibrations always lead to destruction of the cutting edge.

These working instructions have been compiled to the best of our knowledge, but no liability can be accepted for any errors.

5. WELDING

When welding high grade stainless steels & nickel-base alloys, the following instructions should be adhered to:

Workplace

The workplace should be in a separate location, well away from the areas where carbon steel fabrication takes place. Maximum cleanliness and avoidance of draughts are paramount.

Auxiliaries, clothing

Clean fine leather gloves and clean working clothes should be used.

Tools and machinery

Tools used for nickel-base alloys and stainless steels must not be used for other materials. Brushes should be made of stainless material. Fabricating and working machinery such as shears, presses or rollers should be fitted with means (felt, cardboard, plastic sheet) of avoiding contamination of the metal with ferrous particles, which can be pressed into the surface and thus lead to corrosion.

Cleaning

Cleaning of the base metal in the weld area (both sides) and of the filler metal (e. g. welding rod) should be carried out with acetone.

Trichlorethylene (TRI), perchlorethylene (PER), and carbon tetrachloride (TETRA) must not be used.

Edge preparation

This should preferably be done by mechanical means, i. e. turning, milling or planing; plasma cutting is also possible. However, in the latter case the cut edge (the face to be welded) must be finished off cleanly. Careful grinding without overheating is permissible.

Welding process

Different alloy applies different welding processes.