Although welding a bolt isn’t an ideal solution to many engineering problems, sometimes it may be necessary. With the different kinds of bolts available and the various projects they are used for, it is important to know if welding a bolt will weaken it.
Does welding a bolt weaken it? It depends. If the bolt was heat-treated, welding would weaken it because it would heat it beyond its tempering point. In any situation, when considering welding a bolt, you need to know the material properties you’re dealing with beforehand, so you must refer to the bolt’s Mill Test Report or MTR.
While the above answer is short and precise, there is much more information regarding bolts and welding. This article will teach you how to identify weldable bolts and more related information.
How To Identify Weldable Bolts
When heat is applied to a preheated bolt, it’s strength will be altered. Even worse, when a tempered bolt is subjected to heat in an uncontrolled environment, the outcome is uncertain.
This uncertainty is the reason why welding heat-treated bolts is not recommended.
The 14th edition of the AISC (American Institute of Steel Construction) Manual (page 2-25) states, “As a heat-treated material, Grade 105 rods cannot be welded.” This fact illustrates that any high strength, heat-treated bolt shouldn’t be welded.
But what if you aren’t dealing with a heat-treated bolt? Before attempting to weld the bolt, check its Mill Test Report (MTR) and apply the chemistry to assess the weldability of the material.
Even if you find out that your bolt is weldable, the application of the bolt welding procedure matters a great deal. Always consult with an expert near you, especially if dealing with a safety-sensitive project.
What Happens When You Overheat A Steel Bolt?
Overheating a hardened steel bolt can soften it if the temperature used is too high, and the bolt is exposed to the heat for a long time.
Therefore, if you weld a Grade 8 bolt, you could soften it to the strength that Grade 5 possesses.
No matter how little heat you use, you’re heating the bolt in uncontrolled conditions, and it will weaken it.
Understanding Varied Bolt Compositions
When dealing with bolts, you’ll come across different situations that call for various bolt strengths. To get a general idea of the weldability of particular bolts, there are some things you need to know.
Why are High Strength Bolts Heat-Treated?
High strength bolts are heat treated to handle the intense tension and sheer force that are caused by specific loads.
Heat-treated bolts are essential in automotive applications since tensile and force stresses are prevalent.
That’s why you’ll mostly find heat-treated U bolts in off-road cars, heavy-duty trucks, and forest trailers, among others.
What is the Heat Treatment Process for Bolts?
Although many people associate heat treatment with hardening, it’s not always the case. Bolt heat treatment processes differ to achieve specific outcomes.
For example, the quenching and tempering processes will vary depending on the end material desired for a particular purpose.
Quenching alters the original mechanical properties of a metal by heating it to an incredibly high temperature and then cooling it rapidly.
Tempering improves the quality of a metal by heating it with high temperatures and then letting it cool slowly, most often in the open air.
Some heat treatment processes harden the bolt while others soften it. The annealing process, for instance, softens the bolt to reduce its residual stress to boost its ductility.
So, if you get two bolts that state their exact compositions (hardened and softened), you can determine the welding outcome for the bolts.
How to Read the Mill Test Report
The MTR shows you everything you need to know before doing any operations with a particular steel bolt.
It explains the chemical and mechanical specifications according to the legalities in the steel industry.
The Heat/Melt Number
Each bolt has a heat number that traces its mechanical and chemical properties back to the manufacturer.
The heat number can source the processing conditions of a particular batch of steel bolts to assess its working conditions.
Chemical and Mechanical Composition
Assessing the chemical and mechanical properties of bolts is important. It gives you a general idea of the do’s and don’ts for particular bolts. For example, if you have to weld a bolt that produces harmful fumes because of its composition, you can determine how to combat that.
Chemical properties will depend on the aspects of steel type differentiation. These differentiations can be boron, carbon, alloy, or ferritic, among others.
Mechanical properties should detail the bolt’s ductility, torsional strength, fatigue strength, and hardness, among others. The determination of these mechanical abilities is achieved through destructive testing.
Here’s a step by step guide for reviewing an MTR:
- Check out the physical look of the bolt and confirm its properties in the MTR to match the bolt with your needs.
- Look up the chemical composition table and take note of the significant properties you need to know.
- Review the mechanical properties and determine if they are suitable for your application. Note that going for high-grade bolts isn’t the solution to many fastening problems.
- If you are satisfied with what you see, go ahead and use the bolt.
Alternative Processes for Joining Metals
While you might think that your only option to join metals is to weld the bolt to make a connection, check out the following alternatives to see if one of them may work for your project.
Although force is required during riveting, you won’t change the chemical properties of the material you’ll be joining.
If applicable, the rivets will offer you the luxury of achieving a semi-permanent or permanent bond — whichever suits your preferences.
Since brazing involves a filler metal or alloy that melts without the parts you’re joining being affected, it serves as the perfect process in many applications. Even better, it’s quick, and it can join varied thicknesses of metals.
Other Common Bolt Welding Questions Answered
You may still have some questions regarding the welding of bolts. Some of the most frequently asked questions about this process are answered below.
Q. Is welding stronger than bolting?
A. Welding is generally considered to be stronger than bolting since it provides an even distribution of pressure and unifies parts to make one permanently. This conclusion only applies if the right type of welding was done correctly in a particular application.
Since bolting provides a semi-permanent bond that rests pressure quite unevenly, it doesn’t spark confidence in some applications. However, welding can be stronger than bolting at times and impossible in others. It all depends on the use.
Q. How can I loosen a welded bolt?
A. Depending on the nature of the weld, you can choose to drill the bolt out or use a screw extractor. You can saw through the nut gently, then remove the welding material. The screwdriver method will require you to cut a shallow hole to access the bolt’s head. All this will likely word on a mild tack weld.
Q. How can I select bolt sizes?
A. The main factor you must consider when selecting a bolt is finding a safe type for the particular application at a cost that matches the life of the machine. You should make a detailed outline of the specifics of the bolt, and BOLTCALC can help you with that.
Q. Can I use a high strength bolt with a mild steel nut?
A. Matching the right grade of bolt and nut is necessary if you want to make a reliable design. The tensile differences of unmatched grades will not only negatively affect the fastening but also the adaptation to environmental conditions. These effects can result in a disastrous outcome.
Should You Weld A Bolt?
After reading about bolts and welding, you may wonder if welding a bolt is the best decision to make.
It is recommended that you look into your situation and decide if welding a bolt is the only way you think you would make a proper connection. If not, explore other alternatives and make bolt welding your last resort.