Welding defect detection is still often performed by the human visual inspection or with non destructive tests. These quality inspections methods can be time consuming and can have an important error rate. In this paper, we propose an approach for the detection of welding faults through the detection of abnormal subsequences of the welding voltage signal. The approach is based on the One-Class SVM with distance substitution kernels. The One Class-SVM has been used in many works for the detection of abnormal subsequences. However, often after transforming the subsequences into a set of features. Nevertheless, finding the relevant features for anomaly detection may be challenging. Dealing with the raw subsequences in distance-based approaches, on the other hand, are known to be effective and can generalize well to different problems but they often suffer from high computational cost, which may restrict their application, especially with the need of real time predictions in an industrial context. We show in this work that the One-Class SVM can be successfully used directly with the raw subsequences. This is achieved by employing distance substitution kernels. These class of kernels has not yet gained widespread adoption for time series anomaly detection. The results show that the approach is both accurate and fast, which makes it more suitable for real-time welding monitoring. We further propose an approach for automatic diagnosis of welding defects.