New high strength steels will require knowledge of how thermal processes, such as welding, will affect mechanical properties.

MCDP Staff Report

(Click here to see the story as it appears in January/February 2014's Metal Casting Design & Purchasing.)

Eglin steel is an ultra-high strength steel alloy developed at Eglin Air Force Base in Florida in the early 2000s and patented in 2009. It was developed for use in high strain rate applications, such as missile components, penetrating ordnance and armor plating. Precipitation hardened (PH) maraging stainless steel alloys also are being studied for their ultra-high strength. 

These steel alloys have the potential to make a considerable impact on the market. But the effects of certain thermal-based processes, such as casting, heat treatment and welding, on mechanical properties have not been studied closely for control purposes. Researchers Brett Leister, Erin Barrick and John DuPont of the Department of Materials Science and Engineering at Lehigh University, Bethlehem, Pa., set out to produce a continuous cooling diagram for Eglin steel as well as develop heat treat schedules for the PH steels. Their work is described in the paper “Phase Transformations and Welding of Ultra-High Strength Steels,” which was presented at the Steel Founders’ Society of America’s 2013 Technical & Operating Conference.

Question

How do various processing steps affect the mechanical properties of Eglin steel and specific PH maraging stainless steels?

1. Eglin Steel Background

Eglin steel exhibits similar strength levels to AerMet100, AF1410 and HP9-4-30 alloys, currently used for similar applications, but it has a reduced cost due to a reduction in expensive alloying elements such as nickel and cobalt (Table 1). Silicon is added to Eglin steel to enhance toughness. Chromium provides increased strength and hardenability, molybdenum increases hardenability and nickel increases toughness. The addition of tungsten also increases strength. 

Fabricating this steel through casting, forging, fusion welding and heat treatment involves a wide range of cooling rates that can produce various microstructures and resultant properties. Continuous cooling transformation diagrams are helpful for controlling the microstructure and properties, but one has not been developed for Eglin steel. Additionally, Eglin steel weld properties have not been measured and optimized, which can limit the use of this new alloy.

A continuous cooling transformation diagram provides valuable information on the phases that will form during general processing procedures, such as heat treatment, fusion welding and casting, which is useful for optimizing microstructure and properties. Understanding the variation in weld properties in the fusion zone and the heat affected zone will allow proper selection and manipulation of welding parameters to ensure sound welds with optimal properties. The researchers set out to develop a continuous cooling transformation diagram for Eglin steel and develop preliminary processing guidelines for welding to retain the mechanical properties of the weld. 

PH maraging stainless steel alloys have a martensitic matrix. The composition of the three PH alloys used in the study can be found in Table 2. These alloys undergo an aging heat treatment to increase strength. The aging forms precipitates in the matrix. PH17-4 and PH17-4+Co undergo copper precipitation, and PH13-8+Mo undergoes β-NiAl precipitation to increase strength levels in the material. It is not yet known in detail how fusion welding processes affect the properties of these cast alloys, so researchers also set out to establish potential hardness variations within fusion welds made on these PH alloys and develop post-weld heat treatment schedules that can be used to eliminate any soft zones within the welds. 

2. Eglin Steel Phase Transformations Procedure

In addition to developing a continuous cooling transformation diagram for Eglin steel, weld thermal simulations were performed to correlate mechanical properties to microstructure changes in the heat affected zone. For comparison, as-cast and cast and heat treated Eglin steel underwent mechanical testing, as well. Dilatometry and heat affected zone simulations were performed on a Gleeble thermo-mechanical simulator. Autogenous gas tungsten arc welding passes were performed on each of the PH alloys in different heat treat conditions and with a variety of post-weld heat treatments (Tables 3-4). 

The Eglin dilatometry experiments showed four distinct regions form within Eglin steel depending on the cooling rate (Fig. 1). At cooling rates above 1C/second, a martensitic microstructure was formed with a hardness of about 520HV. Intermediate cooling rates (1C/second to 0.2C/second) produced a material with a mixed martensitic/bainitic microstructure with a hardness that ranges from 520 to 420HV. Slower cooling rates (0.1C/second to 0.03C/second) led to the formation of bainitic microstructures with a hardness of about 420HV. The slowest cooling rates of 0.01C/second formed a bainitic microstructure with pearlite at the prior austenite grain boundaries. 

3. Results and Conclusions

Through various studies on phase transformation of Eglin steel, a general continuous cooling transformation diagram was developed for use with forging, casting, rolling, fusion welding and heat treating (Fig. 2). Together with plots of hardness as a function of cooling rate and the microstructural investigation, the following conclusions were made:

• Cooling rates greater than 0.3C/second lead to the formation of a primarily martensitic microstructure.
• The formation of a martensitic microstructure at these higher cooling rates leads to Vickers microhardness values in excess of 520HV.
• Formation of bainite decreases the martensitic start temperature and the overall hardness of the material.
• A second hardness plateau of about 420HV is formed at cooling rates 0.1-0.03C/second that form a bainitic microstructure.
• The formation of pearlite further decreases the hardness, and the pearlite is formed at the prior austenitic grain boundaries in lieu of ferrite because the eutectoid composition has decreased due to the presence of alloying elements. 

Following mechanical testing of heat affected zone simulations performed on Eglin steel, these conclusions were made:

• Heat input does not have a significant effect on the mechanical properties of heat affected zone thermal simulations.
• Strength decreases in the subcritical, intercriticial and fine-grain heat affected zones relative to the base metal, probably due to tempering and carbide coarsening as a result of the thermal cycle. 
• The yield strength of the coarse-grain heat affected zone is increased relative to the base metal, but the tensile strength and ductility are decreased, most likely due to the formation of untempered martensite.• A similar loss of toughness was seen throughout all regions of the heat affected zone in comparison to the base metal.
• The base metal has a mixed mode of fracture consisting of microvoid coalescence and cleavage. The subcritical and intercritical heat affected zones exhibit a fracture surface consisting of only cleavage, whereas the fine-grain and coarse-grain heat affected zones exhibit a mixed mode of fracture of microvoid coalescence and cleavage. 
• Heat treatment following casting is shown to yield similar mechanical properties to the heat treated wrought alloy.
• Toughness of the cast and heat treated Eglin steel is less than its wrought counterpart in all regions of the heat affected zone. 
• Performing the full Eglin steel heat treatment following thermal simulation yields an increase in toughness in all heat affected zone conditions, but does not restore properties to that of the base metal. 

After preliminary welding experiments performed on the PH maraging alloys, the following conclusions were made:

• The hardness variations in the fusion zone and heat affected zone are most likely caused by the presence of precipitates which can be affected by the weld thermal cycle causing coarsening or dissolution.
• Post-weld heat treatments increased hardness in the fusion zone and eliminated drops in hardness in the heat affected zone, most likely due to re-precipitation.  ■

This article is based on a paper presented at the Steel Founders Society of America’s 67th Technical and Operating Conference in Chicago December 2013.