1. Introduction
Identical materials can have different mechanical properties (strength, hardness, plasticity and toughness), and different materials can have similar mechanical properties. All this is closely related to the heat treatment of steel.
Steel parts obtain a certain structure through heat treatment to achieve the required performance characteristics. Heat treatment is a means, achieving performance is the goal, and the structure forms the basis and guarantee of these properties.
Using 45# steel as an example, this section will share the relationship between annealing, normalizing, the structure of 45# steel and its associated properties.
2. Case Analysis
Case 1
- Material name: 45 steel
- Processing status: raw material (supply status, hot rolled)
- Corrosive Agent: 4% Nitric Acid Alcohol Solution
- Microstructural description: The microstructure of a cross-section of a φ13.2 mm bar material is shown in Figure 1. The white ferrite in the figure appears blocky, network-shaped, and needle-shaped, while the pearlite is in fine lamellar layers. The hardness is around 18 HRC, which is comparable to the hardness value after normalization.
The raw material in the supplied state has been hot-rolled and air-cooled, equivalent to normalization, so it is harder than annealed steel.
However, due to the high temperature, some ferrite precipitates along the needle-like grain boundaries and extends into the grain, forming a Widmanstätten structure.
The appearance of the Widmanstätten structure significantly reduces the impact resistance of the steel and makes it brittle. Steel with large grain sizes is especially prone to forming Widmanstätten structures.
To eliminate the Widmanstätten structure and large grains, it is necessary to carry out a normalization treatment before tempering to refine the grain and improve the structure.
Case 2
- Material name: 45 steel
- Processing State: Raw Material (Supplied State, Cut)
- Conditioner: 4% alcohol solution with nitric acid
Note on microstructure: After cutting the φ13.2mm round rod material on a normal cutting machine, the microstructure of the cross-section of the heat-affected zone as shown in Figure 2 was formed due to lack of timely cooling of the water.
The left half of the image represents the original microstructure, while the right half represents the microstructure of the heat-affected zone. The hardness variation in the heat-affected zone is quite significant, ranging between 25-40 HRC.
Figure 3 demonstrates the enlarged microstructure of each zone. Figure 3a represents the structure of Zone 1 in Figure 2.
The left half of the figure shows the original structure of the material, characterized by white net-like ferrite and fine pearlite flakes. The right half shows the structure of the heat-affected zone during cutting, consisting of white polygonal ferrite, pearlite flakes, grayish-white martensite, and residual austenite.
Figure 3b illustrates the structure of Zone 2 in Figure 2, showing undissolved white ferrite at grain boundaries, grayish-white martensite, residual austenite, and thin flakes of pearlite. The thin, dark pearlite flakes within the grains represent a newly formed transition zone microstructure during the shear cooling process.
Figure 3c presents the structure of Zone 3 in Figure 2, similar to an underheated quench structure. Grain boundaries show white polygonal undissolved ferrite, along with grayish-white martensite and residual austenite. Ferrite limits are clearly defined.
During the sample cutting process, varying cutting speeds and feed rates, along with inadequate cooling, resulted in distinct regions of a jewel-blue oxidation layer on the sample surface, as illustrated in Figure 4.
As can be inferred from the figure, there are areas affected by heat during the later stages of cutting. The harder the material, the more challenging the cut and the larger the heat-affected zone becomes.
The first three samples in Figure 4 are high-carbon, high-alloy steel, while the last five are 45# steel.
Before heat treatment, the surfaces of the samples were not fully polished, leading to the observation of varying microstructures in the raw material during examination.
During sample cutting, if cooling is not carried out promptly, the friction between the sample and the grinding wheel causes the sample temperature to rise rapidly to between Ac1 and Ac3 as the cutting speed gradually increases.
After water cooling, a structure similar to underheated quenching is formed. As the surface temperature varies in different regions of the sample, the microstructures in these regions also differ.
Case 3
- Material name: 45 steel
- Processing status: isolation at 830°C for 15 minutes followed by oven cooling (annealing)
- Conditioner: 4% nitric alcohol solution
- Microstructure description: As depicted in Figure 5, the normal microstructure of annealed 45 steel is shown; consists of irregular polygonal ferrite in white and dark lamellar pearlite. The pearlite lamellae are clearly visible, with hardness values ranging from 8 to 11 HRC.
Annealing of 45 steel involves heating the steel above Ac3 by 30-50°C, followed by furnace cooling to allow the steel to acclimatize. This relatively slower cooling process results in a nearly balanced microstructure, with pearlite occupying approximately 55% of the entire visual field area.
Case 4
- Material name: 45 steel
- Treatment status: Air-cooled after being held at 830°C for 15 minutes (normalizing)
- Conditioner: 4% alcohol solution with nitric acid
- Structural Description: As shown in Figure 6, the normal structure of standard 45 steel; composed of white block and ferrite in network and perlite in dark flakes. Pearlite occupies around 70% of the visual field area, with a hardness between 15-20 HRC.
Normalized 45 steel involves heating the steel above its Ac3 temperature by 30-50°C and then allowing it to cool naturally in air. The main difference between this and a full annealing process is the faster cooling rate and greater degree of supercooling.
This results in a finer pearlite lamellar structure compared to annealed steel, with a significant increase in pearlite quantity and relatively smaller grain size. Therefore, the hardness of normalized steel is higher than that of annealed steel.
Normalizing 45 steel can improve its structure after casting or forging by refining the austenite grains and forming fine and uniform ferrite and pearlite, thereby increasing the strength, hardness and toughness of the steel.
Steel 45, with its high strength and good plasticity, can be used in the manufacture of many important components, such as compressors, chemical pumps and moving parts (crankshafts, connecting rods, connecting rods). It can also be used to make turbine blades. Generally, large-sized components are used in a normalized state, while small-sized components can be quenched to form a quenched sorbite.
Steel 45 is also the most commonly used quenched and tempered steel. Before quenching and tempering at high temperature, it must be subjected to a normalization process to obtain a uniform and finely structured organization, preparing the steel for quenching.
3. Conclusion
This article shares the microstructural characteristics of 45 steel in different states. We can appreciate the mystery and charm of heat treatment, as different treatment methods can intelligently change the structure and properties of the material.
45 steel is a commonly used hardened steel. This article analyzes this material in different states, providing a methodical approach from which we believe everyone can gain some insights.
In everyday work, when developing samples for other materials, it would be ideal if we could apply what we learned from one example to others.
