One of the most frequently asked questions by vacuum furnace operators is “Why are my parts discolored?” While many customers may believe they’ve thoroughly tested and inspected their equipment before reaching out to us, there are often areas that might be overlooked during the inspection process.
Here is a list of diagnostic questions you should expect us to ask whenever we get this call:
Initial Data Collection
- What are the cold and hot linear leak rates of the furnace?
- Does the furnace reach the desired process vacuum levels and remain within specifications throughout the cycle?
Defining the Discoloration
- What shows signs of discoloration: the hot zone, the parts, or both?
- What color are you seeing on the parts or hot zone? (green, blue, white, black, etc.)
Physical Inspection
- What is the age and condition of the current hot zone?
- Are there any abnormal odors noticeable in the hot zone when the furnace door is opened?
- Are there any signs of arcing heating elements and/or melted materials within the hot zone?
- Are the cold walls in the furnace clean or dirty?
- What is the condition of the furnace door seal?
- Have you inspected and tested the pumping system to ensure it is functioning properly and obtaining the specified vacuum levels? Are all of the pump oils at the correct level and normal appearance?
Recent Maintenance
- Has there been any scheduled or unplanned maintenance recently performed on the furnace?
- When was the last time the heat exchanger was inspected and/or serviced?
- When did the furnace last undergo a cleanup cycle? How often are cleanup cycles performed on the furnace? What are the temperatures, times and pressures used during the cleanup cycle?
Process Review
- Have the materials, alloys, or recipes recently changed?
- Has the cleaning process for upstream parts, baskets and/or fixtures recently changed? Have the baskets and fixtures been properly maintained and pass inspection?
- Are the baskets, grids and/or racks used in other processes/equipment besides this vacuum furnace?
Leak Detection
- Is the leak detector connected to the pumping system? Where on the system is it connected?
- Have you verified that the inert gas supply lines are leak free?
Diagnostic Testing
- If you run a process that does not utilize partial pressure, followed by a vacuum cool at the end of the cycle, does the furnace yield clean parts?
- Have you wrapped a test part in titanium or stainless-steel foil (depending on the process temperature) to see whether it is the part or the furnace environment that is generating the contamination?
- Has a sample of the contaminated surface been sent to the lab for a trace element review? What were the findings of the report that was generated?
As you go through this diagnostic checklist, the more answers you can provide, the quicker our experts will be able to help you uncover the solution.
If you find yourself working through this diagnostic list and you decide you need help along the way, reach out to us at 844-Go-Ipsen or by emailing technical@ipsenusa.com. We can offer tips and tricks, or you can make an appointment with one of our field service technicians to help you troubleshoot the issues and get your furnace back into production in a timely manner.
What Discoloration Tells Us
Yellow (from straw yellow to golden):
When using argon, yellow parts signify an air or gas leak that invites atmospheric air to mix with argon. The more significant the leak, the darker the yellow coloration gets. Past a point of leak severity, parts may emerge blue instead of yellow.
Blue (from sky blue to midnight blue):
While using argon, blue parts signify increasingly severe air leaks, gas leaks, or atmospheric mixing. While using nitrogen, blue parts signify smaller air leaks. Once again, the darker the color, the more severe the leak is. Blue may also indicate, copper contamination.
Green (from lime to forest green):
Green parts signify a few potential problems, including:
1. The furnace door was left open in-between cycles, or the furnace was not kept under vacuum for an extended period
2. Parts were put into the furnace while excessively wet or with wet braze alloys
3. The furnace was run at high temperatures while in an intense vacuum, liberating chrome from the parts and fixtures
4. The hot zone absorbed water vapor
5. There may have been a copper contamination in the process
6. Water jacket leaks, heat exchanger leaks, traces of water, etc.
Black (from light gray to pitch black):
While using argon, black may indicate a severe leak in the system. Black may also indicate carbon contamination in the process. This may happen during partial pressure carburizing, or be the result of sooty parts that haven’t been cleaned completely before thermal processing.
*Contamination layers less than 0.1 μm are transparent, and human eyes cannot distinguish the color correctly. Customers can identify discoloration based on the thickness of the contamination layer in the heating chamber or the part. Tool steels are less sensitive and can be heat-treated under a low vacuum level. Stainless steel and super alloys are susceptible to even the slightest traces of oxygen in the vacuum system.