brazing heat treatment

We’ve assembled some of the top 101 Heat Treat Tips that heat treating professionals submitted over the last three years into today’s original content. If you want more, search for “101 heat treat tips” on the website! Today’s tips will remind you of the importance of materials science and chemistry.

By the way, Heat Treat Today introduced Heat Treat Resources last year; this is a feature you can use when you’re at the plant or on the road. Check out the digital edition of the September Tradeshow magazine to check it out yourself!

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Induction Hardening Cast Iron

Induction hardening of cast irons has many similarities with hardening of steels; at the same time, there are specific features that should be addressed. Unlike steels, different types of cast irons may have similar chemical composition but substantially different response to induction hardening. In steels, the carbon content is fixed by chemistry and, upon austenitization, cannot exceed this fixed value. In contrast, in cast irons, there is a “reserve” of carbon in the primary (eutectic) graphite particles. The presence of those graphite particles and the ability of carbon to diffuse into the matrix at temperatures of austenite phase can potentially cause the process variability, because it may produce a localized deviation in an amount of carbon dissolved in the austenitic matrix. This could affect the obtained hardness level and pattern upon quenching. Thus, among other factors, the success in induction hardening of cast irons and its repeatability is greatly affected by a potential variation of matrix carbon content in terms of prior microstructure. If, for some reason, cast iron does not respond to induction hardening in an expected way, then one of the first steps in determining the root cause for such behavior is to make sure that the cast iron has not only the proper chemical composition but matrix as well.

(Dr. Valery Rudnev, FASM, Fellow IFHTSE, Professor Induction, Director Science & Technology, Inductoheat Inc.)

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14 Quench Oil Selection Tips

Here are a few of the important factors to consider when selecting a quench oil. 

1. Part Material – chemistry & hardenability 
2. Part loading – fixturing, girds, baskets, part spacing, etc. 
3. Part geometry and mass – thin parts, thick parts, large changes in section size 
4. Distortion characteristics of the part (as a function of loading) 
5. Stress state from prior (manufacturing) operations 
6. Oil type – characteristics, cooling curve data 
7. Oil speed – fast, medium, slow, or marquench  
8. Oil temperature and maximum rate of rise 
9. Agitation – agitators (fixed or variable speed) or pumps 
10. Effective quench tank volume 
11. Quench tank design factors, including number of agitators or pumps, location of agitators, size of agitators, propellor size (diameter, clearance in draft tube), internal tank baffling (draft tubes, directional flow vanes, etc.), flow direction, quench elevator design (flow restrictions), volume of oil, type of agitator (fixed v. 2 speed v. variable speed), maximum (design) temperature rise, and heat exchanger type, size, heat removal rate in BTU/hr & instantaneous BTU/minute.
12. Height of oil over the load 
13. Required flow velocity through the workload 
14. Post heat treat operations (if any) 

(Dan Herring, “The Heat Treat Doctor®”, of The HERRING GROUP, Inc.)

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How to Achieve a Good Braze

In vacuum brazing, be certain the faying surfaces are clean, close and parallel. This ensures the capillary action needed for a good braze.

A good brazing filler metal should:

1. Be able to wet and make a strong bond on the base metal on which it’s to be applied.
2. Have suitable melt and flow capabilities to permit the necessary capillary action.
3. Have a well-blended stable chemistry, with minimal separation in the liquid state.
4. Produce a good braze joint to meet the strength and corrosion requirements.
5. Depending on the requirements, be able to produce or avoid base metal filler metal interactions.

(ECM USA)

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Pay Attention to Material Chemistry

When trying to determine a materials response to heat treatment, it is important to understand its form (e.g., bar, plate, wire, forging, etc.), prior treatments (e.g. mill anneal, mill normalize), chemical composition, grain size, hardenability, and perhaps even the mechanical properties of the heat of steel from which production parts will be manufactured. The material certification sheet supplies this basic information, and it is important to know what these documents are and how to interpret them.

Certain alloying elements have a strong influence on both the response to heat treatment and the ability of the product to perform its intended function. For example, boron in a composition range of 0.0005% to 0.003% is a common addition to fastener steels. It is extremely effective as a hardening agent and impacts hardenability. It does not adversely affect the formability or machinability. Boron permits the use of lower carbon content steels with improved formability and machinability.

During the steelmaking process, failure to tie up the free nitrogen results in the formation of boron nitrides that will prevent the boron from being available for hardening. Titanium and/or aluminum are added for this purpose. It is important, therefore, that the mill carefully controls the titanium/nitrogen ratio. Both titanium and aluminum tend to reduce machinability of the steel, however, the formability typically improves. Boron content in excess of 0.003% has a detrimental effect on impact strength due to grain boundary precipitation.

Since the material certification sheets are based on the entire heat of steel, it is always useful to have an outside laboratory do a full material chemistry (including trace elements) on your incoming raw material. For example, certain trace elements (e.g. titanium, niobium, and aluminum) may retard carburization. In addition, mount and look at the microstructure of the incoming raw material as an indicator of potential heat treat problems.

(Dan Herring, The Heat Treat Doctor^®)

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Aqueous Quenchant Selection Tips

Determine your quench: Induction or Immersion? Different aqueous quenchants will provide either faster or slower cooling depending upon induction or immersion quenching applications. It is important to select the proper quenchant to meet required metallurgical properties for the application.

1. Part material: Chemistry and hardenability are important for the critical cooling rate for the application.
2. Part material: Minimum and maximum section thickness is required to select the proper aqueous quenchant and concentration.
3. Select the correct aqueous quenchant for the application as there are different chemistries. Choosing the correct aqueous quenchant will provide the required metallurgical properties.
4. Review selected aqueous quenchant for physical characteristics and cooling curve data at respective concentrations.
5. Filtration is important for aqueous quenchants to keep the solution as clean as possible.
6. Check concentration of aqueous quenchant via kinematic viscosity, refractometer, or Greenlight Unit. Concentration should be monitored on a regular basis to ensure the quenchant’s heat extraction capabilities.
7. Check for contamination (hydraulic oil, etc.) which can have an adverse effect on the products cooling curves and possibly affect metallurgical properties.
8. Check pH to ensure proper corrosion protection on parts and equipment.
9. Check microbiologicals which can foul the aqueous quenchant causing unpleasant odors in the quench tank and working environment. If necessary utilize a biostable aqueous quenchant.
10. Implement a proactive maintenance program from your supplier.

(Quaker Houghton)

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Container Clarity Counts!

Assure that container label wording (specifically for identifying chemical contents) matches the corresponding safety data sheets (SDS). Obvious? I have seen situations where the label wording was legible and accurate and there was a matching safety data sheet for the contents, but there was still a problem. The SDS could not be readily located, as it was filed under a chemical synonym, or it was filed under a chemical name, whereas the container displayed a brand name. A few companies label each container with (for instance) a bold number that is set within a large, colored dot. The number refers to the exact corresponding SDS.

(Rick Kaletsky)

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Check out these magazines to see where these tips were first featured: