Study On Healing Condition And Method Of Internal Defect Of Large Forgings

Large forgings, the fundamental parts for heavy machine manufacturing, are very important to the national economy, defense and the momentous equipments with advanced science and technology. Their productivity and quality represent the level of nation's industrialization. With the current status of research and manufacturing about forgings in the world are concerned, two theoretical problems about large forgings, which need urgently resolution, are focused on in this thesis-to set up the condition for eliminating defects in large forgings and find the way to recover large forgings having defects.Porosity and void are two main types of unavoidable defects in steel castings, and recovering conditions are established respectively for them, according to their different mechanical and deformation features.The porosity defect is that in certain area inside the cast ingot exist small cellular voids relatively equally distributed, slight difference in mechanical properties, obvious plastic volume deformation and variation of relative density governed by the spherical stress tensor. Consequently an analytical equation in terms of porosity, strain and spherical stress tensor is put forward by virtue of logarithmic strain, the plastic condition and the physical equation of pore mechanics. Further more the porosity crushing condition for forging's recovering is presented.Void defect means that voids occur in certain portion of the ingots. The size of voids are much larger than that of porosity but still are negligible as compared to the dimensions of the ingots, so an approximated mechanical model of void, finite-dimension in an infinite continuum, can be used. Therefore the theory of damage mechanics is employed in the research on void to get the condition for eliminating the defects. For linear viscous materials, the analytical equation expressed by loads and void's deformation is obtained with micro-damage mechanics, however for nonlinear viscous materials the semi-analytical equation is obtained with continuous damage mechanics. The results from calculation agree with those from numerical simulation introduced in the references.Stretching is a main approach for void crushing in large forging process and the shape of anvil is the major and the most easily controlled factor affecting the recovering. By analyzing the stress and deformation, a new type of anvil, called concave anvil, ispresented. The results from numerical caculation shown that the stress and strain in defect zone under reduction deformation using this anvil are more preferable than those using ordinary anvils.Also in this thesis, the porosity recovering condition in CFAF and CVAF processes is verified by experiments with specimen of lead of viscous-flow, and the results confirm the advantages of the anvil and the correctness of the close-recovering theory. The void compression recovering condition is also verified with specimen of lead powder, and the theoretical correctness of the condition is confirmed.Using the research results, the current technology for forgings of 330 MW low-pressure rotors is analyzed and a new technology is recommended.