What low expansion addresses
Reference points in a measuring fixture should remain as stable as possible when ambient temperature changes. In composite tooling, the relative dimensional response of the mould and component matters. Invar 36, an iron-nickel material with approximately 36 percent nickel, is considered for such uses. The selection priority is low expansion over a defined temperature interval, rather than general stainless behaviour or maximum hardness.
The “36” designation does not define every physical property. Delivery condition, processing history and the intended temperature interval also matter. For tight tolerances, do not equate stock quality with the dimensional stability of the completed assembly; connections and other components also move with temperature.
Read expansion coefficients with a temperature interval
Linear dimensional change can be approximated by multiplying initial length, mean expansion coefficient and temperature difference. This shows why even a small coefficient can produce measurable movement in a long component. The coefficient is not constant at all temperatures, however. Read a tabulated value with its starting and ending temperatures; averages over different intervals are not directly comparable.
A fixture measured near room temperature and a mould entering a hot process do not use the same interval. Temperature differences between regions can matter as much as total length change. A structure that is hot on one side and cool on the other may suffer alignment or flatness problems even when its average temperature appears acceptable.
Stabilisation is not the same objective as hardening
The objective for a precision Invar component is predictable dimensions in subsequent use. Material removal during rough machining can change the residual-stress balance. Roughing, stabilisation or stress relief and finishing may therefore be planned as separate stages. The treatment depends on geometry and required precision; the grade name alone does not define one universal sequence.
These treatments should not be confused with those used to age harden a high-strength superalloy. Thermal history planned for dimensional stability has a different purpose from increasing hardness. Establish the dimensional inspection stage and whether welding, straightening or further forming will take place afterwards.
Assess the completed assembly
When bolts, bushes or supports made from another alloy are attached to an Invar body, differential expansion can affect contact loads and alignment. Fully constraining a connection is not always beneficial. Consider the permitted directions of movement, the measurement datum and how thoroughly temperature is allowed to equalise.
Surface and storage conditions also affect precision use. Nickel content does not justify treating Invar 36 like stainless steel. Choose protection, cleaning and measuring surfaces for the application. Recording the temperatures of both the component and measuring equipment makes acceptance measurements more meaningfully comparable.
Before measurement, bringing only the outer surface to ambient temperature may be insufficient. Thick bodies and thin projections equalise at different rates. Using a consistent waiting period and setup for repeated measurements helps distinguish material behaviour from measurement conditions, avoiding an assessment of stabilisation based on a misleading temperature transition.
Enquiry information for precision fabrication
Rather than requesting only “Invar plate”, explain the conditions in which stability is required. Check UNS references found in different records against the intended product specification; similar names do not make all delivery requirements identical.
- Operating and measurement temperature intervals, target tolerance and reference length.
- Rough and final dimensions, material removal and connection geometry.
- Planned stabilisation, welding or final-forming operations.
- Required expansion-testing and dimensional-acceptance records.