Cold Forming Vs Hot Forging Vs Machined Steel Bolts and Screws
Cold Forming, Hot Forging, and Machining are all common methods of making fasteners. Today we’re going to take a look at each method and point out some benefits and drawbacks to each one.
Cold-Formed Bolt with a Machined Head – Sometimes more than one process is necessary.
Cold forming, or cold heading, is the process of forming a bolt or screw without heating up the material. This is accomplished by striking or pressing the part within a form, or die, in order to create the desired dimensions. Many times, more than one strike is required. Hot forging, on the other hand, involves heating up the steel to a point where it becomes malleable and then forming it in a die. The third method of making bolts and screws that we’ll be talking about today is machining. A machined part is shaped by cutting away material. Depending on the part, each of these three methods can be the preferred technique. I’ll examine a few common issues and discuss the advantages and disadvantages of each method.
Size and Shape of the Part
A disadvantage of cold forming is that there is a limitation to the size of parts that you can make. The bigger the diameter of a bolt or screw, the more pressure is needed to push the material into the desired shape. Given a big enough machine, you could make as big a part as you wanted. But at some point, the sheer size of the machine needed to cold form the steel becomes impractical. For this reason, larger diameter parts are typically hot forged. Heating the steel beyond its recrystallization point causes the steel to become considerably more ductile, thereby allowing the parts to be formed using much less pressure.
Additionally, cold forming has inherent limitations to what shapes can be formed. Innovations in technology have vastly improved the capabilities of cold forming, but even today, some parts can be extremely difficult or even impossible to form without heating or cutting the material. In contrast, complicated parts can be machined relatively easily, making this the preferred method for many intricately shaped parts. Or, as another alternative, heating steel increases the ductility of the material, which allows for the hot forging of parts where cold heading won’t work.
Bin full of scrap from machining some studs
A major benefit of a cold-formed or a hot-forged part over a machined part is the lack of scrap. When you machine a part, you achieve the desired shape by removing material. With cold heading and hot heading, you are forming the material into the shape that you want, generally without removing any material. Cold forming also has an advantage over hot forming because no additional energy is required to heat the steel.
Strength
Another benefit to cold forming over machining or hot heading is that the process of cold forming adds strength to the part in a process called work hardening. Because the material is moved without being cut or heated, the grain structure of the steel remains intact. The grains flow with the contours of the part, adding strength. When you machine a part, grain flow and work hardening do not occur. Hot forging keeps some of the steel grains intact, but the strengthening effect is limited because heating steel modifies the grain structure.
Material
The type of material you are using will have a large impact on which forming method you choose. Different metals and alloys behave differently. Some, such as alloy steel, are ductile enough at room temperature to cold form easily. Other materials, like low carbon steel, benefit from the added strength from cold forming.
However, there are plenty of materials that do not lend themselves to cold forming very well. For example, certain types of stainless steel can be quite difficult to form properly at room temperature. Or there are cases where the work hardening imparted by cold forming can restrict the amount of material flow, making it difficult to achieve the desired shape. Sometimes, it’s just easier to machine a part rather than struggle to form it properly. Some types of aluminum, for example, lend themselves quite well to machining. Finally, there are materials that do not cold form or machine very well and need to be heated to high temperatures in order to form properly. Different materials call for different methods.
All three of these methods are commonly used to make bolts and screws. The key to success is in knowing which method to employ in a given situation.
