Materials

INVAR

 

 1. 36% nickel/iron alloys

 

To help achieve very high stability mechanics alloys of the generic name of Invar may be useful.

An iron alloy with a low coefficient of thermal expansion was invented in 1896 by Charles Edouard Guillaume. It was named “invar” because of its “invariable effect”. A number of companies produce a low CTE alloy with about 36% nickel. Each company has trademarked their own name for this alloy.

Trademarked   Name

Manufacturer

Invar 36 &   super Invar 32-5

Carpenter   Technology Corp

Pernifer 36

ThseenKrupp-VDM

Invar, Inovar,   Microvar & Inovco

Arcelor Mittal

Nilo 36

Special Metals   Corporation

Table 1- 36% nickel/iron alloys

These alloys are designed to have a zero CTE at 20°C with the CTE increasing above this temperature. Typical quoted values are between 0.6 and 1.4 x 10-6 /K between 20 and 100°C. Super Invar which is manufactured by Carpenter Technology has a coefficient which increases at a lower rate than Invar 36 TM, as shown in Figure 1. Heat treatment is required to obtain the best stability from these alloys.

                       

 

Figure 1–CTE of  Invar TM and Super Invar TM

 

But what is the best heat treatment process? The following sources provide advice

2. Heat treatment process options

Alloy wire web site

Anneal at 830°C (1525°F) for 30 minutes and water quench, re-heat to 300°C (570°F) for 1 hour and water quench. Finally re-heat to 100°C (212°F) for 48 hours and air cool.

Eagle alloys corporation web site

Annealing Method 1
Heat parts to 1525°F (815° C)+- 25°F and hold at temperature one-half hour per inch of thickness, then furnace cool at a rate not to exceed 200°F per hour to 600°F. No additional machining should be performed on these parts

Annealing Method 2
1. Rough Machine
2. Heat parts to 1525°F (815°C) +- 25°F and hold at temperature one-half hour per inch of thickness, then furnace cool at a rate not to exceed 200°F per hour to 600°F. Still air cool is acceptable below 600°F
4. Heat Parts for one hour at 600"F (300°C) +- 20°F followed by air cooling
5. Heat parts for 48 hours at 205°F followed by air cooling
6. Finish Machine.

Annealing Method 3 - Annealing plus water quench and stabilization method
1. Rough Machine
2. Heat parts to 1525°F (815°C)+- 25°F and hold at temperature one-half hour per inch of thickness, then water quench
3. Semi finish machine
4. Heat Parts for one hour at 600"F (300°C) +- 20°F followed by air cooling
5. Heat parts for 48 hours at 205°F (81°C)  followed by air cooling
6. Finish Machine.

 

 

High temp metals web site

Full Anneal after extensive forming 1hr at 1550°F or 3 hours at 1350°F air or oven cool

Stress relieve between rough and final machining. 2 hours at 600°F air or oven cool

Cartech Web site

Heat Treatment for Optimal Dimensional Stability

The presence of cold work stresses causes very slight changes in dimensional stability with respect to time and temperature. This change can be detected only with exceedingly sensitive devices.

To assure optimal dimensional stability, heat to 1500°F (815°C), hold at heat for 30 minutes per inch of thickness, water quench, reheat to 600°F (315°C) holding one hour at heat, then air cool.

To promote temporal stability (when necessary), Carpenter Invar "36" alloy has been aged for 24 to 48 hours at 200°F (93°C)

Annealing

Heat to 1450°F (790°C) and hold at heat 30 minutes per inch of thickness, then air cool. Heating to temperatures above 1000°F (538°C) relieves the presence of cold work stresses.

Espimetals web site

Anneal : The alloy softens progressively when heated in the range of 1000 to 2300 oF. Pronounced grain growth does no occur until 1900 oF has been passed. It can be air cooled or water quenched from the annealing temperature.

Stress Relieve : Heat to 600 - 700 oF for about one hour, air cool, reheat to a temperature somewhat above the top operating temperature, cool slowly to somewhat below the lower operating temperature, again heat slowly to above the operating temperature, cool slowly to room temperature (cooling very slowly through the Curie temperature is also considered to improve stability).

 

References

Article- Accurate Clock Pendulums Robert J. Matthys, Chapter 22 The heat treatment of invar

Publisher:: Oxford University Press, DOI:10.1093/acprof:oso/9780198529712.003.0022

Even though invar is the most common material used for the pendulum rod in a good clock, it is still a poor material for the purpose because of its relatively poor dimensional stability over time compared with other materials such as quartz or platinum. Invar is usually considered for its low thermal expansion coefficient (tempco) rather than its dimensional stability. Quartz, however, is an ideal material for a pendulum rod, if you can get around the glass breakage problem. There are three types of invar available: regular invar, regular invar free machining, and super invar. Each has a different tempco and is dependent on heat treatment and any cold working or machining that the part has received. What never gets mentioned and is not widely known is how big the changes from heat treating and machining really are.

 

Invited paper PIE Viol 1335 Dimensional stability (1990). Unstable optics, S F Jacobs

A graph is presented of temporal stability of Invars after various heat treatments but this is coped from another paper and no details of the heat treatment processes are given. ”for bulk material the temporal stability is not yet readily obtainable”

Article- Dimensional stability of high purity Invar 36 – Sokolowski et al. JPL

High purity (Carbon and other impurities <0.01%) Invar 36 made by powder metallurgy is an exceptionally dimensional stable material. Temporal stability <1ppm/year was achieved. The materials tested all included 11 weeks at 38C. A variety of heat treatment processes for High purity Invar were tested. The best 3 step process was as follows

Annealing at 788°C/ 30 min slow cool

Stress relief at 316°C/ 1 hr

Aging at 93°C/ 48 hours.

A simple 2 step process also gave good results. Stress relive at 316°C/1 hour. Aging at 93°C/48 hours.

Article – Effects of temporal dimensional stability on the Advanced X-ray astrophysics facility high resolution mirror assembly.  L. M Cohen.

The EKC method provides a much more uniform stability than does the MIT method.

The EKC method was solution heat treat at 830°C with water quench

100°C slow oven cool with another 100°C cycle after hand lapping.

Article – Temperature and age effects on the temporal stability of Invar. J.M.Steele

Temporal stability of Invar is characterised by a combination  of short (8-12 ppm/year over 100days)  and long (3-8 years) term behaviour. Long term growth is most likely due to carbon migration. Short term rapid growth is most likely caused by residual stress relaxation from stresses not removed during the 400F stress relieving heat treatment.

An initial rate( 1 year after heat treatment) of 3.5ppm/year with a 3 year time constant provides a reasonable estimate of the long term behaviour of Invar at ambient conditions.

The most significant parameters affecting the temporal stability of Invar are

Letter – Applied Optics-Dimensional stability of Invars. S.F.Jacobs et al.

There are at present no commercial source of Invar or super Invar that is known to be ultrastable.  Our results underscore the wisdom of aging Invar before use.

Email from Les Harmer Consultant to Ed Fagan Inc. 

A three step heat treat procedure is generally suggested to provide low thermal expansivity and dimensional stability for the most demanding applications:

 1.  830oC – 1/2 hour – water quench

2.  315oC – 1 hour – air cool

3.  96oC - 24 hours – air cool

 Some information is available in ASTM F 1684 (in the appendix) on expansivity performance with several heat treatments but not temporal stability measurements. 

 The procedure is not always used.  The first step can cause warpage due to rapid cooling.  It can also cause cracking, particularly for sections greater than 2.5 cm in thickness or diameter.

 Temporal stability is known to be associated with carbon ageing and step 3 will accelerate this process.  Carbon levels are much lower than they were years ago with the development of better melting practices.  Expansivity changes due to temporal stability are known to be very small and difficult to measure/ detect.  I haven't seen much published on the subject.

 Some people who have a concern might just perform step 3.

 

Pernifer 36 data sheet

The nominally low values of thermal expansion up to 100°C are attained by using the following 3-step process

830°C, 3min, water quench

Reheat to 300°C, one hour, cooling in air

Reheat to 100°C, hold for 30min, furnace cool for 48 hours to room temperature.

 

Nilo 36 Data sheet

For most uses annealing at 850-90°0C, slow cooling

For highest dimensional stability with minimum expansion

830°C for 30mins. Water quench, reheat to 300°C air cool

Reheat to 100°C for 48 hours air cool.

Book- Physics and applications of invar alloys, Honda memorial series on materials science No 3, 1978, Maruzen Company Ltd, Tokyo.

Chapter 21 – stabilization processes to obtain good dimensional stability and a minimum coefficient of thermal expansion may be given as follows:

830°C, 30min water quench

315°C, 1 hr, air cool

95°C, 48hr, air cool

3. Summary

There seems to be general agreements that temporal stability of Invar at room temperature can be reduced by the following

The heat treatment recommended varies. A common suggestion is the three step process with a water quench, but three steps with air cool and 2 step methods are also suggested. A common is however the ageing at 96°C for at least 24 hours.

It is suggested that on drawings we state the following

 

Option 1 Heat treatment

830oC – 1 hour – water quench

315oC – 1 hour – air cool

96oC - 24 hours – air cool

Option 2 Heat treatment

315oC – 1 hour – air cool

96oC - 24 hours – air cool

Material

36% Nickel  Iron Alloy

<0.01% Carbon

 

A coefficient of thermal expansion at 20°C of about 1.3 x 10-6 seems a reasonable assumption.