Steel Alloys

The main attribute of steel is its high strength. Steel is also a ductile material, meaning that it can deformed elastically. Even though it has a low strength-to-weight ratio, it is chosen for many applications in which weight is not as important as safety and durability. Structural members used for buildings and heavy machinery are made of steel alloys.

Steel alloys are durable, strong, they resist high impact forces with ease, and many alloying elements are added to improve its machinability, durability, hardness, and other mechanical properties.

Another important fact is that steel is inexpensive. Recycling methods applied nowadays have dropped the prices of steel to a point in which 1 Kg of steel costs an average of 0,20$. Its low price is another reason why it is better for those applications in which weight is not a problem. Thanks to the recycling methods and the abundance of iron, steel alloys have an extraordinary availability.

Steel is classified depending on its composition or the way that it was processed. Based on their composition, steels are categorized as plain carbon steels, alloy steels, and specialty steels.

Plain carbon steels are those composed of approximately 2% carbon or less. This category also includes: High-carbon steels have carbon above 0,6%, medium-carbon steels range from 0,3% to 0,6%, mild steels have between 0,15% < C < 0,3%, low-carbon steels with a composition ranging between 0,04% and 0,15% of C, ultra-low carbon with a maximum of 0,03%, and de-carburized steels with less than 0,005%. These steels may also have small amounts of other alloying elements such as manganese, silicon, and copper.

Alloy steels have a higher amount of alloying element, enhancing its mechanical properties and improving their manufacture.  In alloy steels, the maximum concentration of carbon is 1% and the alloying elements are up to 50%. Steel is also considered alloy steel if it has titanium, chrome, molybdenum, or nickel included in its alloying elements.

Specialty steels are those improved by heat-treatment or have relatively high concentrations of certain alloying elements to boost their mechanical properties.

An example of specialty steels is the tool steels. Tool steels are usually high-carbon heat-treated to improve their properties. They’re used to manufacture cutting tools for CNC machining, casting dies, forming dies, and other applications in which a combination of toughness, strength, and temperature resistance is required.

Stainless steels are also considered specialty steels due to their high concentration of chromium, which may be as high as 30%. Chromium is what makes stainless steels stainless, improving its corrosion resistance

Here we are going to focus on specialty tool steels, but if you’d like to know more about stainless steels, you can check this link (add the link to stainless steels here)

1. ASTM 228 Steel

Also known as music steel for its usage for piano strings, ASTM 228 categorized as high-carbon steel having a composition ranging from 97,8 to 99% of iron, between 0,7 and 1% of carbon. ASTM 228 alloyed with small concentrations of manganese, silicon, sulfur, and phosphorus too. It has a high fatigue resistance, being the main reason why it’s mostly used on high-quality and small springs, piano strings, and other applications that need high stress and fatigue resistance.

√ Mechanical Properties

√ Manufacturing Processes Performance

2. SAE/AISI Grade 4140 Steel

This alloy steel uses chromium and molybdenum as alloying elements to increase its mechanical properties. The use of Cr and Mo benefits both strength and fatigue resistance, making this one of the favorite steels used for tubular chassis in the automotive industry. It has a very good machinability rating due to the use of sulfur and acceptable corrosion and wears resistance thanks to the addition of chromium and molybdenum. Its mechanical properties and processing characteristics are shown for the alloy after being oil-quenched at 540°C.

√ Mechanical Properties

√ Manufacturing Processes Performance

3. AISI Type W1 Tool Steel

Hardened by water quenching, the W1 tool style is acknowledged for its low price and mechanical properties. Composed of a 0,70 to 1,5% carbon range, silicon, chromium, nickel, vanadium, and tungsten. Mostly used for cutting tools, its properties guarantee impressive durability and enough hardness to machine most aluminum and steel alloys.

√ Mechanical Properties

√ Manufacturing Processes Performance

4. SAE/AISI 1010 Steel

Mostly used in applications where low loads are applied to the element, this low-carbon steel it’s appropriate to be used on magnet cores, transmission covers, fasteners, and bolts. Being a plain carbon steel, it is only composed by 0,08 to 0,13% of carbon, 99% of iron, and small concentrations of sulfur and phosphorus to improve its machinability. This steel has very good machinability but low strength and fatigue resistance. Its poor hardness makes it vulnerable to excessive wearing if not properly lubricated.

√ Mechanical Properties

√ Manufacturing Processes Performance

Surface finishing processes developed over carbon steels are inclined to improve their corrosion resistance and to smoothen the surface. Rough surfaces are susceptible to augment the oxidation mechanisms that tend to corrode a metal’s surface. The most commonly finishing methods applied are:

1. Surface machining

manual or CNC performed, mechanical finishing is applied by a lathing, milling, or a grinding process. Executing these processes over a rough surface tends to smoothen it, diminishing the effects of corrosion and allowing it to achieve better tolerances. Mechanical or electro-polishing used to increase metal anti-corrosive properties.

2.Sandblasting

sandblasting or abrasive-finishing is another technique used to both remove corrosion stains and smoothen the surface of a machine component. The main benefit of sandblasting over the regular machining processes mentioned above is that it is easier to set up for complex geometries.

3.Painting

Surface painting creates a protective layer on the metal’s surface, keeping it away from the corrosion mechanisms that may affect it. The main two issues related to surface painting is that it requires a machining or sandblasting process to achieve a smooth surface to ensure a proper finish, and the protective layer also has low impact resistance.

Steel alloy is the most significant engineering material not only because it was one of the most used during the industrial revolution, but because it is still used in most common applications for its high strength and ductility. Here we present some tips for those who want to add steel pieces to their projects:

1. Steel has the lowest price compared to every strong metal, but it has a considerable weight. Steel is reserved for applications where weight is not as important as mechanical resistance.

2. Carbon steels have poor corrosion resistance. When using any kind of non-stainless carbon steel in highly corrosive environments, it is recommended to protect the surface by painting it.

3. Most carbon steels have a good machinability that combined with their impressive strength makes the machining process a good option to achieve difficult geometries.

4. Consider using cold-drawn finishing, it improves the mechanical properties of steel making plain carbon steel as strong as an alloyed one, reducing the costs of your product.