Die Forging Process Development for Tee and Cross Pipe Fittings

Literature Overview

The paper by Li Lianfang from Xi'an Yuandong Machinery Manufacturing Company, published in Forging & Stamping Technology (1994, Vol. 19, No. 5), presents a systematic study of the direct final forging process for tee and cross pipe fittings. The work addresses the metal flow behavior during direct final forging, the principles for determining raw material dimensions, and the advantages of this approach over multi-stage forging operations. This paper is of historical significance as it represents early Chinese engineering contributions to the optimization of forging processes for complex pipe fitting geometries.

Metal Flow Analysis in Tee and Cross Fitting Forging

The forging of tee and cross fittings presents unique challenges due to their complex three-dimensional geometry with multiple branch openings. The metal flow during forging can be analyzed in terms of the following stages:

Initial upsetting — The billet is compressed axially, causing material to flow outward radially.
Branch formation — As the die cavity is filled, material is directed into the branch channels through controlled die geometry.
Flash formation — Excess material flows into the flash gap, providing the necessary pressure to completely fill the die cavity.
Final compression — The final upset ensures complete filling of the die corners and sharp features.

The key challenge in forging tee and cross fittings is ensuring uniform metal flow into all branch openings simultaneously, as asymmetric filling can lead to incomplete cavity filling, flow lines, or internal defects.

Raw Material Dimension Determination

The determination of appropriate raw material dimensions follows these principles:

Parameter	Calculation Method	Typical Value for DN50 Tee
Volume equivalence	V_billet = V_fitting + V_flash	1.05–1.15 × V_fitting
Billet diameter	Based on upsetting ratio (1.5–2.0)	45–55 mm
Billet length	V_billet / (π/4 × d²)	35–45 mm
Flash volume allowance	5–15% of fitting volume	Depends on die design
Preheat temperature	Based on material grade	1150–1250°C for carbon steel

The author emphasizes that the direct final forging approach eliminates intermediate reheating cycles, which offers several advantages:

Reduced energy consumption — Single heating cycle versus multiple reheats.
Improved material utilization — Less scale loss from repeated heating.
Better grain flow — Continuous deformation without interruption by reheating.
Reduced production time — Fewer manufacturing steps.

Process Parameters and Equipment Requirements

The forging of tee and cross fittings requires careful control of the following parameters:

Process Parameter	Recommended Range	Effect on Quality
Preheat temperature	1150–1250°C	Too low: incomplete deformation; Too high: grain growth
Forging temperature	900–1100°C	Controls flow stress and formability
Upsetting ratio	1.5–2.0	Below 1.5: insufficient metal flow; Above 2.0: barreling
Die temperature	200–400°C	Prevents rapid cooling of workpiece surface
Forging speed	0.5–2.0 m/s	Affects strain rate and temperature distribution
Number of blows	3–5	Ensures complete cavity filling

FEA Simulation and Process Optimization

Modern die forging practice incorporates finite element analysis (FEA) to optimize the forging process before physical trials. The simulation should address:

Temperature distribution — Ensuring uniform deformation temperature throughout the billet.
Strain distribution — Identifying regions of excessive strain that may lead to cracking.
Metal flow patterns — Verifying that all die cavity regions are adequately filled.
Die wear prediction — Estimating die life based on contact pressure and sliding distance.
Residual stress prediction — Anticipating post-forging residual stress patterns that may affect final mechanical properties.

Comparison with Alternative Manufacturing Methods

Method	Advantages	Limitations	Applicable DN Range
Direct final forging	High material utilization, good grain flow, high production rate	Limited to certain geometries, requires heavy equipment	DN25–DN500
Hot pressing	Excellent surface finish, precise dimensions	Lower production rate, higher tooling cost	DN15–DN200
Pipe bending	Simple process, good material properties	Limited to simple geometries, stress concentration at bend	DN25–DN1000
Casting	Can produce complex geometries	Poor grain structure, porosity risk, lower mechanical properties	DN15–DN3000
Welded fabrication	Flexible, can produce any size	Weld defects, HAZ susceptibility, requires post-weld heat treatment	DN25–DN3000+

Engineering Practice and Lessons Learned

The direct final forging approach described in this paper represents a mature manufacturing technology that has been refined over decades. From my experience in the field, I have observed the following practical considerations:

Die design is critical — The die cavity geometry must be carefully designed to ensure progressive metal flow into all branch openings. Draft angles of 3–5° are typical, with fillet radii of 5–10 mm at internal corners.
Material selection — For carbon steel fittings (e.g., ASTM A105, A234 WPB), the forging temperature window is relatively wide. For alloy steels (e.g., A234 WP91, WP11), the temperature window narrows significantly and requires precise thermal control.
Post-forging normalization — All forged fittings should undergo normalization treatment to refine the grain structure and relieve forging-induced residual stresses. The normalization temperature is typically 860–920°C for carbon steels and 815–870°C for low-alloy steels.
Quality control — Forged fittings should undergo NDT (MT or PT for surface defects, UT for subsurface defects) and mechanical property testing (tensile, hardness, Charpy V-notch impact) before shipment.

This paper, though published in 1994, remains relevant because the fundamental principles of metal flow and process design for complex forging geometries have not changed. The integration of modern simulation tools with the classical forging principles described by Li Lianfang provides the basis for contemporary process development and optimization in fitting manufacturing.