Impact of cutting fluid on hybrid manufacturing of AISI H13 tool steel

Purpose This paper aims to analyse the effects derived from the presence of residual coolant from machining operations on the Directed Energy Deposition of AISI H13 tool steel and the quality of the resulting part. Design/methodology/approach In the present paper, the effectiveness of various cleaning techniques, including laser vaporising and air blasting, applied to different water/oil concentrations are studied. For this purpose, single-layer and multi-layer depositions are performed. Besides, the influence of the powder adhered to the coolant residues remaining on the surface of the workpiece is analysed. In all cases, cross-sections are studied in-depth, including metallographic, microhardness, scanning electron microscopy and crack mechanism analyses. Findings The results show that, although no significant differences were found for low oil concentrations when remarkably high oil concentrations were used the deposited material cracked, regardless of the cleaning technique applied. The crack initiation and propagation mechanisms have been analysed, concluding that the presence of oil leads to hydrogen induced cracking. Originality/value High oil concentration residues from previous machining operations in hybrid manufacturing led to hydrogen induced cracking when working with AISI H13 tool steel. The results obtained will help in defining future hybrid manufacturing processes that combine additive and subtractive operations.

Hydrogen-Induced Cracking Caused by Galvanic Corrosion of Steel Weld in a Sour Environment

This study examined the hydrogen-induced cracking (HIC) caused by galvanic corrosion of an ASTM A516-65 steel weld in a wet sour environment using a combination of standard immersion corrosion test, electrochemical analyses, and morphological observation of corrosion damage. This study showed that the weld metal has lower open circuit potential, and higher anodic and cathodic reaction rates than the base metal. The preferential dissolution and much higher density of localized corrosion damage were observed in the weld metal of the welded steel. On the other hand, the presence of weldment can make steel more susceptible to HIC, specifically, in areas of the base metal but not in the weld metal or heat affected zone, which is in contrast to typical expectations based on metallurgical knowledge. This can be explained by galvanic corrosion interactions between the weldment and the base metal, acting as a small anode and a large cathode, respectively. This type of galvanic couple can provide large surface areas for infusing cathodically-reduced hydrogen on the base metal in wet sour environments, increasing the susceptibility of welded steel to HIC.

Hydrogen Induced Cracking Damage Estimation and Evaluation

New non-destructive testing (NDT) inspection technology, quantification of damage, and tensile testing, has enabled the assessment of conservatism associated with the hydrogen induced cracking (HIC) damage parameter (DH) currently used in API 579 – 1/ASME Fitness-For-Service (FFS) Part 7. To address HIC damage from an FFS perspective, the general requirements include addressing protection against plastic collapse and protection against cracking. The focus of this body of work is only to address conservatism regarding the protections against plastic collapse. The detrimental effects of HIC damage on plastic collapse is modeled through the DH parameter, currently set at 0.8 or an 80% strength loss in HIC damage material for the Level 2 HIC Assessment Procedure. Original development of the DH parameter was based on tensile testing of HIC damaged material performed in air, where the HIC damage was not sized or quantified, and a 30% margin was added to the maximum measured reduction in tensile strength to get to the 80% strength loss. Modification of the DH parameter is allowed in the Level 3 HIC Assessment Procedure, provided supporting testing data justifying a reduction is also provided with the assessment. For the tensile testing in air, the quantified HIC damage and tensile testing results are consistent with an 80% strength loss, without an added margin. A rigorous ultrasonic testing inspection using conventional phased array ultrasonic testing (PAUT) and PAUT with full matrix capture using the total focusing method (FMC/TFM) was performed on ex-service SA-212 Grade B material. Locations with service generated HIC damage were extracted and tensile tested in air and in gaseous hydrogen. Examination of the tensile specimen fracture surfaces allowed for quantification of HIC damage associated with final tensile failure. HIC damage measured with NDT was similar to the HIC damage on the fracture surface when characterized using the crack sensitivity ratio (CSR). The hydrogen tensile testing results suggested that for material still charged with hydrogen (not currently explicitly addressed in API 579 – 1/ASME FFS Part 7), the loss in strength may be larger than 80%.