Improvement of Forging Process for Tee Fittings

Literature Overview

This paper by Ge Fusheng (1989), published in Forging and Stamping Technology, describes the improvement of a forging process for a tee fitting used in an injection molding machine. The fitting is manufactured from A3 carbon steel (equivalent to modern Q235) and is produced in small batches, leading the author to select free forging as the manufacturing method. The paper presents the forging die design, the forging sequence, and the process improvements implemented to enhance productivity and part quality.

Material and Product Requirements

The tee fitting in question is a structural component in an injection molding machine, subjected to moderate mechanical loads and pressure loads from the hydraulic system. The A3 steel (Q235) selection is appropriate for this application, providing adequate strength, toughness, and weldability at a reasonable cost. The forging process is selected over casting because:

Forged parts exhibit superior mechanical properties due to the refinement of the grain structure and the elimination of casting defects such as porosity, shrinkage cavities, and segregation.
The fibrous grain flow pattern in forged parts follows the contour of the component, providing enhanced fatigue resistance at stress concentration locations.
For small batch production, free forging with simple tooling is more economical than investment casting or precision forging.

The key mechanical properties expected for A3 steel after forging and normalizing are:

Property	Typical Value	Test Standard
Tensile strength (Rm)	375-500 MPa	GB/T 228
Yield strength (Re)	≥ 235 MPa	GB/T 228
Elongation (A5)	≥ 26%	GB/T 228
Impact energy (KV2)	≥ 27 J at 20°C	GB/T 229
Hardness	125-197 HBW	GB/T 231

Forging Process Design and Improvement

The original forging process for the tee fitting involved conventional free forging operations on a mechanical power hammer or hydraulic press. The improved process incorporates several key modifications:

Preheating optimization: The billet is preheated to a forging temperature of 1100-1150°C, with a holding time of 15-20 minutes per 25 mm of diameter to ensure uniform temperature distribution throughout the cross-section. This eliminates the temperature gradient that causes surface cracking and internal folding during heavy deformation.
Forging sequence refinement: The forging sequence is optimized to minimize the number of operations while ensuring adequate metal flow at each stage. The sequence typically includes: upsetting (to increase the cross-sectional area), drawing (to reduce the cross-section and increase length), bending (to form the branch angle), and final shaping (to define the tee geometry).
Die design improvement: The die blocks are designed with appropriate draft angles (typically 2-5°) to facilitate part removal, and the die cavity geometry is optimized to promote uniform metal flow and minimize folding. The die material is selected as tool steel (e.g., 5CrMnMo or 3Cr2W8V) with appropriate heat treatment to achieve a surface hardness of 48-52 HRC.
Cooling rate control: After forging, the part is cooled at a controlled rate to prevent quench cracking and minimize residual stress. For A3 steel, air cooling in still air is typically sufficient, but for thicker sections (> 50 mm), furnace cooling or buried cooling in dry sand may be required.
Heat treatment specification: The forged tee is normalized at 850-880°C followed by air cooling to refine the grain structure and improve mechanical properties. For components subject to fatigue loading, a tempering treatment at 550-600°C may be applied after normalizing to further reduce residual stress.

Quality Control and Defect Prevention

The forging process for tee fittings is susceptible to several characteristic defects that must be controlled:

Defect Type	Cause	Detection Method	Prevention Measure
Surface cracks	Excessive deformation rate; low forging temperature	Visual inspection; MT	Preheat optimization; controlled deformation rate
Internal folding	Improper die geometry; excessive upsetting ratio	UT; MT after machining	Die design review; limit upsetting ratio to 1.5-2.0
Incomplete filling	Insufficient forging force; excessive cooling	Visual inspection; dimensional check	Increase forging temperature; reduce cooling rate
Grain coarsening	Excessive forging temperature; prolonged holding	Metallographic examination	Limit forging temperature to 1150°C; minimize holding time
Segregation	Inadequate billet quality; insufficient deformation	Spectroscopic analysis; MT	Source control of billet; ensure adequate reduction ratio

Engineering Practice and Process Improvement Philosophy

The approach described in this paper reflects a systematic process improvement methodology that is directly applicable to modern forging operations. The key principles are:

Start with the product requirements: The mechanical properties, dimensional tolerances, and surface quality requirements of the tee fitting dictate the forging process parameters.
Optimize the process sequence: Each forging operation should be designed to maximize metal flow efficiency while minimizing the risk of defects.
Control the thermal cycle: The heating, forging, and cooling temperatures must be carefully controlled to achieve the desired microstructure and mechanical properties.
Implement quality control at every stage: From incoming material inspection to final mechanical property testing, quality control checkpoints must be established at each stage of the process.

For modern forging operations, additional improvements include the use of numerical simulation (DEFORM, FORGE, or AutoForm) to predict metal flow and identify potential defect locations before trial production. The integration of process monitoring systems for real-time tracking of forging temperature, deformation rate, and die temperature can further enhance process consistency and quality.

Study Insights and Reflections

This paper, though published in 1989 and addressing a relatively simple forging application, embodies the fundamental principles of forging process engineering that remain relevant today. The emphasis on preheating optimization, die design, and controlled cooling reflects a deep understanding of the metallurgical behavior of steel during hot working. The process improvement approach — identifying the root causes of quality problems and implementing targeted corrective actions — is a methodology that transcends the specific application and applies to all manufacturing processes. For engineers involved in the design and manufacture of pipe fittings, the forging process is one of several available manufacturing routes, and the selection between forging, casting, welding, or machining should be based on a comprehensive evaluation of cost, quality, and performance requirements. The tee fitting described in this paper is a simple example, but the principles of forging process optimization — thermal control, metal flow management, and defect prevention — are directly transferable to the forging of more complex pipe fittings such as large-diameter reducers, flanges, and high-pressure valve bodies.