Fig. 1. Influence of alloying elements on grain-growth behavior (20MnCr5) [1]

As heat treaters, we are always looking for ways to shorten our processing times. Increasing carburizing temperature has long been known to shorten cycle times, but grain growth has been our Achilles’ heel. Today there is renewed focus on optimization of diffusion-related processes, and certain microalloying elements show promise for allowing us to raise carburizing temperatures. Let’s learn more.

What Is Grain Growth, and Why Do We Care?

Grain growth, or an increase in the size of the grains, occurs by the movement of grain boundaries. A grain boundary is simply the interface between two adjacent grains. Grain boundaries can be thought of as imperfections in the crystal structure acting as barri-ers to the movement of dislocations (defects). Since these boundaries are regions of high energy, they make excellent sites for the nucleation of precipitates and contribute to the formation of secondary phases – such as martensite – in steel. As such, they influ-ence mechanical properties – finer-grained steels have higher yield strength and do not harden quite as deeply and as a result have fewer tendencies to crack. Grain boundaries are also preferred sites for the onset of corrosion, and they influence the mechanism of creep.

It has long been held that grain size will remain small for steels held at temperatures slightly above the critical temperature, but at higher temperatures a wide variation in grain size occurs depending on chemical composition and the deoxi-dation method used in the steelmaking practice. Today microalloy element additions of aluminum, niobium and titanium (Fig. 1) have been found to form fine precipitates (carbonitrides), and it is now possible to produce case-hardening steels that are resistant to grain growth up to temperatures as high as 2100°F (1150°C).[1]

These small carbonitride particles act to prevent the movement of grain boundaries by exerting a “pinning” pressure. Zener pinning is the influence of a dispersion of fine particles on the movement of grain boundaries and has a strong influence on recovery, recrystallization and grain growth.

Some Experimental Results

The grain-coarsening behavior of a modified 16MnCr5 (5120) steel chemistry (Table 1) has been investigated[2] by looking at the temperature at which precipitates form.

Over the entire temperature range (Fig. 2), steel B, the base composition, showed the lowest amount of precipitates while steel D showed the highest. At 1750°F (950°C), the amount of carbonitride precipitates present reached 236 ppm for steel B, 400 ppm for steel C and 814 ppm for steel D. The composition of the carbonitride precipitates revealed that in steel B only NbC precipitates are formed. In the steels C and D, which have significantly higher N contents, complex carbonitrides of Nb and Ti are formed, with Nb and N as the most prominent constitu-ents.

Fig. 2. Grain growth behavior of microalloyed steels [2]

Whereas Steel B shows undesirable grain growth at 1925°F (1050°C), the other microalloyed steel grades retain their fine-grained structure. Steel A – containing 450 ppm Nb and an increased amount of Al and N – shows the finest grain size distribution (ASTM 10) up to 1925°F (1050°C). Beyond 1925°F (1050°C), grain growth sets in with a 90% cumula-tive frequency of ASTM 1 at 2000°F (1100°C). Steels C and D show significantly less grain coarsening. With 90% of the grains being smaller than ASTM 6, they still remain fine grained up to 2000°F (1100°C).

Due to the increased amount of microalloying elements, the materials have a significant amount of Nb carbides and nitrides as well as Ti nitrides. Besides Al nitrides – that dissolve at temperatures above 1825°F (1000°C) – these precipitates act as grain boundary pinning particles. This gives improved grain size stability at higher tempera-tures that has been confirmed by various grain growth investigations. At temperatures above 2000°F (1100°C), even the Nb and Ti carbonitrides coarsen in a way that allows grain growth to take place.

Influence of Alloying Elements

When niobium is added to steels, it forms very stable carbides that facilitate grain refinement and allow precipitation hardening to achieve the strengthening mechanisms. The addition of as little as 0.02% Nb can increase the yield strength of medium-carbon steel by 70-100 MPa (10-15 ksi). Alloy additions are usually in the range of 0.01%-0.20%. Small additions of niobium increase the yield strength and, to a lesser degree, the tensile strength.

Titanium additions refine the grain size and decrease hardenability. It is also a very strong carbide and nitride former. Titanium steels exhibit secondary hardening upon tempering due to the precipitation of TiC. The effect is increased when austenitizing temperatures are above 1800°F (980°C).

Aluminum has strong grain-refining properties but has a weak affect on hardenability and detracts from deep hardening. Aluminum, tends to enhance creep, and it is widely used as a deoxidizer. Of all the alloying elements, aluminum is the most effective in controlling grain growth prior to quenching.

Nitrogen increases the tendency to form carbonitrides and has a positive effect on hardenability. Nitrogen increases the strength, hardness and machinability of steel but decreases the ductility and toughness. In aluminum-killed steels, nitrogen forms aluminum nitride particles that control the grain size of the steel, thereby improving both toughness and strength. The presence of various alloy nitride particles tends to retain a finer grain size during tempering.

Grain Size Control

Microalloying elements show great promise in preventing undesired grain growth by increasing the grain-coarsening temperature. Some studies[1] suggest that with the addition of microalloying elements such as niobium, aluminum and titanium, it is possible to raise the carburizing temperature before the onset of significant grain growth above 1900°F (1040°C) and possibly as high as 2100°F (1150°C). To achieve a fine grain structure, it appears that additions of at least 300 ppm aluminum, 450 ppm niobium or 100 ppm tita-nium are necessary to steels already containing about 175 ppm nitrogen.

Finally, a more homogeneous grain size and hard-ness distribution is reported to produce a reduction in distortion, reducing post-machining operations and thereby cost.[2] IH